diff --git a/core/management/commands/populate_more.py b/core/management/commands/populate_more.py
index 50a0052f..eaac58c0 100644
--- a/core/management/commands/populate_more.py
+++ b/core/management/commands/populate_more.py
@@ -1,14 +1,12 @@
 import random
 from datetime import date, timedelta
 from datetime import timezone as tz
-from decimal import Decimal
 from typing import Iterator
 
 from dateutil.relativedelta import relativedelta
 from django.conf import settings
 from django.core.management.base import BaseCommand
-from django.db.models import Count, Exists, F, Min, OuterRef, Subquery, Sum
-from django.db.models.functions import Coalesce
+from django.db.models import Count, Exists, Min, OuterRef, Subquery
 from django.utils.timezone import localdate, make_aware, now
 from faker import Faker
 
@@ -268,24 +266,6 @@ class Command(BaseCommand):
         Product.buying_groups.through.objects.bulk_create(buying_groups)
         Counter.products.through.objects.bulk_create(selling_places)
 
-    @staticmethod
-    def _update_balances():
-        customers = Customer.objects.annotate(
-            money_in=Sum(F("refillings__amount"), default=0),
-            money_out=Coalesce(
-                Subquery(
-                    Selling.objects.filter(customer=OuterRef("pk"))
-                    .values("customer_id")  # group by customer
-                    .annotate(res=Sum(F("unit_price") * F("quantity"), default=0))
-                    .values("res")
-                ),
-                Decimal("0"),
-            ),
-        ).annotate(real_balance=F("money_in") - F("money_out"))
-        for c in customers:
-            c.amount = c.real_balance
-        Customer.objects.bulk_update(customers, fields=["amount"])
-
     def create_sales(self, sellers: list[User]):
         customers = list(
             Customer.objects.annotate(
@@ -355,7 +335,7 @@ class Command(BaseCommand):
             sales.extend(this_customer_sales)
         Refilling.objects.bulk_create(reloads)
         Selling.objects.bulk_create(sales)
-        self._update_balances()
+        Customer.objects.update_amount()
 
     def create_permanences(self, sellers: list[User]):
         counters = list(
diff --git a/counter/baker_recipes.py b/counter/baker_recipes.py
new file mode 100644
index 00000000..aa77fb06
--- /dev/null
+++ b/counter/baker_recipes.py
@@ -0,0 +1,18 @@
+from model_bakery.recipe import Recipe, foreign_key
+
+from club.models import Club
+from core.models import User
+from counter.models import Counter, Product, Refilling, Selling
+
+counter_recipe = Recipe(Counter)
+product_recipe = Recipe(Product, club=foreign_key(Recipe(Club)))
+sale_recipe = Recipe(
+    Selling,
+    product=foreign_key(product_recipe),
+    counter=foreign_key(counter_recipe),
+    seller=foreign_key(Recipe(User)),
+    club=foreign_key(Recipe(Club)),
+)
+refill_recipe = Recipe(
+    Refilling, counter=foreign_key(counter_recipe), operator=foreign_key(Recipe(User))
+)
diff --git a/counter/models.py b/counter/models.py
index 1f1c6b51..1666572b 100644
--- a/counter/models.py
+++ b/counter/models.py
@@ -20,14 +20,15 @@ import random
 import string
 from datetime import date, datetime, timedelta
 from datetime import timezone as tz
+from decimal import Decimal
 from typing import Self, Tuple
 
 from dict2xml import dict2xml
 from django.conf import settings
 from django.core.validators import MinLengthValidator
 from django.db import models
-from django.db.models import Exists, F, OuterRef, Q, QuerySet, Sum, Value
-from django.db.models.functions import Concat, Length
+from django.db.models import Exists, F, OuterRef, Q, QuerySet, Subquery, Sum, Value
+from django.db.models.functions import Coalesce, Concat, Length
 from django.forms import ValidationError
 from django.urls import reverse
 from django.utils import timezone
@@ -45,6 +46,39 @@ from sith.settings import SITH_COUNTER_OFFICES, SITH_MAIN_CLUB
 from subscription.models import Subscription
 
 
+class CustomerQuerySet(models.QuerySet):
+    def update_amount(self) -> int:
+        """Update the amount of all customers selected by this queryset.
+
+        The result is given as the sum of all refills minus the sum of all purchases.
+
+        Returns:
+            The number of updated rows.
+
+        Warnings:
+            The execution time of this query grows really quickly.
+            When updating 500 customers, it may take around a second.
+            If you try to update all customers at once, the execution time
+            goes up to tens of seconds.
+            Use this either on a small subset of the `Customer` table,
+            or execute it inside an independent task
+            (like a Celery task or a management command).
+        """
+        money_in = Subquery(
+            Refilling.objects.filter(customer=OuterRef("pk"))
+            .values("customer_id")  # group by customer
+            .annotate(res=Sum(F("amount"), default=0))
+            .values("res")
+        )
+        money_out = Subquery(
+            Selling.objects.filter(customer=OuterRef("pk"))
+            .values("customer_id")
+            .annotate(res=Sum(F("unit_price") * F("quantity"), default=0))
+            .values("res")
+        )
+        return self.update(amount=Coalesce(money_in - money_out, Decimal("0")))
+
+
 class Customer(models.Model):
     """Customer data of a User.
 
@@ -57,6 +91,8 @@ class Customer(models.Model):
     amount = CurrencyField(_("amount"), default=0)
     recorded_products = models.IntegerField(_("recorded product"), default=0)
 
+    objects = CustomerQuerySet.as_manager()
+
     class Meta:
         verbose_name = _("customer")
         verbose_name_plural = _("customers")
@@ -141,18 +177,6 @@ class Customer(models.Model):
         account = cls.objects.create(user=user, account_id=account_id)
         return account, True
 
-    def recompute_amount(self):
-        refillings = self.refillings.aggregate(sum=Sum(F("amount")))["sum"]
-        self.amount = refillings if refillings is not None else 0
-        purchases = (
-            self.buyings.filter(payment_method="SITH_ACCOUNT")
-            .annotate(amount=F("quantity") * F("unit_price"))
-            .aggregate(sum=Sum(F("amount")))
-        )["sum"]
-        if purchases is not None:
-            self.amount -= purchases
-        self.save()
-
     def get_full_url(self):
         return f"https://{settings.SITH_URL}{self.get_absolute_url()}"
 
diff --git a/counter/tests/test_counter.py b/counter/tests/test_counter.py
index f37a25fb..cdc10835 100644
--- a/counter/tests/test_counter.py
+++ b/counter/tests/test_counter.py
@@ -12,15 +12,13 @@
 # OR WITHIN THE LOCAL FILE "LICENSE"
 #
 #
-import json
 import re
-import string
 from datetime import timedelta
 
 import pytest
 from django.conf import settings
 from django.core.cache import cache
-from django.test import Client, TestCase
+from django.test import TestCase
 from django.urls import reverse
 from django.utils import timezone
 from django.utils.timezone import now
@@ -31,7 +29,6 @@ from club.models import Club, Membership
 from core.baker_recipes import subscriber_user
 from core.models import User
 from counter.models import (
-    BillingInfo,
     Counter,
     Customer,
     Permanency,
@@ -313,149 +310,6 @@ class TestCounterStats(TestCase):
         ]
 
 
-@pytest.mark.django_db
-class TestBillingInfo:
-    @pytest.fixture
-    def payload(self):
-        return {
-            "first_name": "Subscribed",
-            "last_name": "User",
-            "address_1": "3, rue de Troyes",
-            "zip_code": "34301",
-            "city": "Sète",
-            "country": "FR",
-            "phone_number": "0612345678",
-        }
-
-    def test_edit_infos(self, client: Client, payload: dict):
-        user = subscriber_user.make()
-        baker.make(BillingInfo, customer=user.customer)
-        client.force_login(user)
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        user.refresh_from_db()
-        infos = BillingInfo.objects.get(customer__user=user)
-        assert response.status_code == 200
-        assert hasattr(user.customer, "billing_infos")
-        assert infos.customer == user.customer
-        for key, val in payload.items():
-            assert getattr(infos, key) == val
-
-    @pytest.mark.parametrize(
-        "user_maker", [subscriber_user.make, lambda: baker.make(User)]
-    )
-    @pytest.mark.django_db
-    def test_create_infos(self, client: Client, user_maker, payload):
-        user = user_maker()
-        client.force_login(user)
-        assert not BillingInfo.objects.filter(customer__user=user).exists()
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == 200
-        user.refresh_from_db()
-        assert hasattr(user, "customer")
-        infos = BillingInfo.objects.get(customer__user=user)
-        assert hasattr(user.customer, "billing_infos")
-        assert infos.customer == user.customer
-        for key, val in payload.items():
-            assert getattr(infos, key) == val
-
-    def test_invalid_data(self, client: Client, payload: dict[str, str]):
-        user = subscriber_user.make()
-        client.force_login(user)
-        # address_1, zip_code and country are missing
-        del payload["city"]
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == 422
-        user.customer.refresh_from_db()
-        assert not hasattr(user.customer, "billing_infos")
-
-    @pytest.mark.parametrize(
-        ("operator_maker", "expected_code"),
-        [
-            (subscriber_user.make, 403),
-            (lambda: baker.make(User), 403),
-            (lambda: baker.make(User, is_superuser=True), 200),
-        ],
-    )
-    def test_edit_other_user(
-        self, client: Client, operator_maker, expected_code: int, payload: dict
-    ):
-        user = subscriber_user.make()
-        client.force_login(operator_maker())
-        baker.make(BillingInfo, customer=user.customer)
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == expected_code
-
-    @pytest.mark.parametrize(
-        "phone_number",
-        ["+33612345678", "0612345678", "06 12 34 56 78", "06-12-34-56-78"],
-    )
-    def test_phone_number_format(
-        self, client: Client, payload: dict, phone_number: str
-    ):
-        """Test that various formats of phone numbers are accepted."""
-        user = subscriber_user.make()
-        client.force_login(user)
-        payload["phone_number"] = phone_number
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == 200
-        infos = BillingInfo.objects.get(customer__user=user)
-        assert infos.phone_number == "0612345678"
-        assert infos.phone_number.country_code == 33
-
-    def test_foreign_phone_number(self, client: Client, payload: dict):
-        """Test that a foreign phone number is accepted."""
-        user = subscriber_user.make()
-        client.force_login(user)
-        payload["phone_number"] = "+49612345678"
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == 200
-        infos = BillingInfo.objects.get(customer__user=user)
-        assert infos.phone_number.as_national == "06123 45678"
-        assert infos.phone_number.country_code == 49
-
-    @pytest.mark.parametrize(
-        "phone_number", ["061234567a", "06 12 34 56", "061234567879", "azertyuiop"]
-    )
-    def test_invalid_phone_number(
-        self, client: Client, payload: dict, phone_number: str
-    ):
-        """Test that invalid phone numbers are rejected."""
-        user = subscriber_user.make()
-        client.force_login(user)
-        payload["phone_number"] = phone_number
-        response = client.put(
-            reverse("api:put_billing_info", args=[user.id]),
-            json.dumps(payload),
-            content_type="application/json",
-        )
-        assert response.status_code == 422
-        assert not BillingInfo.objects.filter(customer__user=user).exists()
-
-
 class TestBarmanConnection(TestCase):
     @classmethod
     def setUpTestData(cls):
@@ -529,341 +383,6 @@ def test_barman_timeout():
         assert bar.barmen_list == []
 
 
-class TestStudentCard(TestCase):
-    """Tests for adding and deleting Stundent Cards
-    Test that an user can be found with it's student card.
-    """
-
-    @classmethod
-    def setUpTestData(cls):
-        cls.krophil = User.objects.get(username="krophil")
-        cls.sli = User.objects.get(username="sli")
-        cls.skia = User.objects.get(username="skia")
-        cls.root = User.objects.get(username="root")
-
-        cls.counter = Counter.objects.get(id=2)
-
-    def setUp(self):
-        # Auto login on counter
-        self.client.post(
-            reverse("counter:login", args=[self.counter.id]),
-            {"username": "krophil", "password": "plop"},
-        )
-
-    def test_search_user_with_student_card(self):
-        response = self.client.post(
-            reverse("counter:details", args=[self.counter.id]),
-            {"code": "9A89B82018B0A0"},
-        )
-
-        assert response.url == reverse(
-            "counter:click",
-            kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-        )
-
-    def test_add_student_card_from_counter(self):
-        # Test card with mixed letters and numbers
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "8B90734A802A8F", "action": "add_student_card"},
-        )
-        self.assertContains(response, text="8B90734A802A8F")
-
-        # Test card with only numbers
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "04786547890123", "action": "add_student_card"},
-        )
-        self.assertContains(response, text="04786547890123")
-
-        # Test card with only letters
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "ABCAAAFAAFAAAB", "action": "add_student_card"},
-        )
-        self.assertContains(response, text="ABCAAAFAAFAAAB")
-
-    def test_add_student_card_from_counter_fail(self):
-        # UID too short
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "8B90734A802A8", "action": "add_student_card"},
-        )
-        self.assertContains(
-            response, text="Ce n'est pas un UID de carte étudiante valide"
-        )
-
-        # UID too long
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "8B90734A802A8FA", "action": "add_student_card"},
-        )
-        self.assertContains(
-            response, text="Ce n'est pas un UID de carte étudiante valide"
-        )
-
-        # Test with already existing card
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "9A89B82018B0A0", "action": "add_student_card"},
-        )
-        self.assertContains(
-            response, text="Ce n'est pas un UID de carte étudiante valide"
-        )
-
-        # Test with lowercase
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "8b90734a802a9f", "action": "add_student_card"},
-        )
-        self.assertContains(
-            response, text="Ce n'est pas un UID de carte étudiante valide"
-        )
-
-        # Test with white spaces
-        response = self.client.post(
-            reverse(
-                "counter:click",
-                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
-            ),
-            {"student_card_uid": "              ", "action": "add_student_card"},
-        )
-        self.assertContains(
-            response, text="Ce n'est pas un UID de carte étudiante valide"
-        )
-
-    def test_delete_student_card_with_owner(self):
-        self.client.force_login(self.sli)
-        self.client.post(
-            reverse(
-                "counter:delete_student_card",
-                kwargs={
-                    "customer_id": self.sli.customer.pk,
-                    "card_id": self.sli.customer.student_cards.first().id,
-                },
-            )
-        )
-        assert not self.sli.customer.student_cards.exists()
-
-    def test_delete_student_card_with_board_member(self):
-        self.client.force_login(self.skia)
-        self.client.post(
-            reverse(
-                "counter:delete_student_card",
-                kwargs={
-                    "customer_id": self.sli.customer.pk,
-                    "card_id": self.sli.customer.student_cards.first().id,
-                },
-            )
-        )
-        assert not self.sli.customer.student_cards.exists()
-
-    def test_delete_student_card_with_root(self):
-        self.client.force_login(self.root)
-        self.client.post(
-            reverse(
-                "counter:delete_student_card",
-                kwargs={
-                    "customer_id": self.sli.customer.pk,
-                    "card_id": self.sli.customer.student_cards.first().id,
-                },
-            )
-        )
-        assert not self.sli.customer.student_cards.exists()
-
-    def test_delete_student_card_fail(self):
-        self.client.force_login(self.krophil)
-        response = self.client.post(
-            reverse(
-                "counter:delete_student_card",
-                kwargs={
-                    "customer_id": self.sli.customer.pk,
-                    "card_id": self.sli.customer.student_cards.first().id,
-                },
-            )
-        )
-        assert response.status_code == 403
-        assert self.sli.customer.student_cards.exists()
-
-    def test_add_student_card_from_user_preferences(self):
-        # Test with owner of the card
-        self.client.force_login(self.sli)
-        self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8F"},
-        )
-
-        response = self.client.get(
-            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
-        )
-        self.assertContains(response, text="8B90734A802A8F")
-
-        # Test with board member
-        self.client.force_login(self.skia)
-        self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8A"},
-        )
-
-        response = self.client.get(
-            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
-        )
-        self.assertContains(response, text="8B90734A802A8A")
-
-        # Test card with only numbers
-        self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "04786547890123"},
-        )
-        response = self.client.get(
-            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
-        )
-        self.assertContains(response, text="04786547890123")
-
-        # Test card with only letters
-        self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "ABCAAAFAAFAAAB"},
-        )
-        response = self.client.get(
-            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
-        )
-        self.assertContains(response, text="ABCAAAFAAFAAAB")
-
-        # Test with root
-        self.client.force_login(self.root)
-        self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8B"},
-        )
-
-        response = self.client.get(
-            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
-        )
-        self.assertContains(response, text="8B90734A802A8B")
-
-    def test_add_student_card_from_user_preferences_fail(self):
-        self.client.force_login(self.sli)
-        # UID too short
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8"},
-        )
-
-        self.assertContains(response, text="Cet UID est invalide")
-
-        # UID too long
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8FA"},
-        )
-        self.assertContains(response, text="Cet UID est invalide")
-
-        # Test with already existing card
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "9A89B82018B0A0"},
-        )
-        self.assertContains(
-            response, text="Un objet Student card avec ce champ Uid existe déjà."
-        )
-
-        # Test with lowercase
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8b90734a802a9f"},
-        )
-        self.assertContains(response, text="Cet UID est invalide")
-
-        # Test with white spaces
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": " " * 14},
-        )
-        self.assertContains(response, text="Cet UID est invalide")
-
-        # Test with unauthorized user
-        self.client.force_login(self.krophil)
-        response = self.client.post(
-            reverse(
-                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
-            ),
-            {"uid": "8B90734A802A8F"},
-        )
-        assert response.status_code == 403
-
-
-class TestCustomerAccountId(TestCase):
-    @classmethod
-    def setUpTestData(cls):
-        cls.user_a = User.objects.create(
-            username="a", password="plop", email="a.a@a.fr"
-        )
-        user_b = User.objects.create(username="b", password="plop", email="b.b@b.fr")
-        user_c = User.objects.create(username="c", password="plop", email="c.c@c.fr")
-        Customer.objects.create(user=cls.user_a, amount=10, account_id="1111a")
-        Customer.objects.create(user=user_b, amount=0, account_id="9999z")
-        Customer.objects.create(user=user_c, amount=0, account_id="12345f")
-
-    def test_create_customer(self):
-        user_d = User.objects.create(username="d", password="plop")
-        customer, created = Customer.get_or_create(user_d)
-        account_id = customer.account_id
-        number = account_id[:-1]
-        assert created is True
-        assert number == "12346"
-        assert len(account_id) == 6
-        assert account_id[-1] in string.ascii_lowercase
-        assert customer.amount == 0
-
-    def test_get_existing_account(self):
-        account, created = Customer.get_or_create(self.user_a)
-        assert created is False
-        assert account.account_id == "1111a"
-        assert account.amount == 10
-
-
 class TestClubCounterClickAccess(TestCase):
     @classmethod
     def setUpTestData(cls):
diff --git a/counter/tests/test_customer.py b/counter/tests/test_customer.py
new file mode 100644
index 00000000..2d7e1c60
--- /dev/null
+++ b/counter/tests/test_customer.py
@@ -0,0 +1,535 @@
+import json
+import string
+
+import pytest
+from django.test import Client, TestCase
+from django.urls import reverse
+from model_bakery import baker
+
+from core.baker_recipes import subscriber_user
+from core.models import User
+from counter.baker_recipes import refill_recipe, sale_recipe
+from counter.models import BillingInfo, Counter, Customer, Refilling, Selling
+
+
+@pytest.mark.django_db
+class TestBillingInfo:
+    @pytest.fixture
+    def payload(self):
+        return {
+            "first_name": "Subscribed",
+            "last_name": "User",
+            "address_1": "3, rue de Troyes",
+            "zip_code": "34301",
+            "city": "Sète",
+            "country": "FR",
+            "phone_number": "0612345678",
+        }
+
+    def test_edit_infos(self, client: Client, payload: dict):
+        user = subscriber_user.make()
+        baker.make(BillingInfo, customer=user.customer)
+        client.force_login(user)
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        user.refresh_from_db()
+        infos = BillingInfo.objects.get(customer__user=user)
+        assert response.status_code == 200
+        assert hasattr(user.customer, "billing_infos")
+        assert infos.customer == user.customer
+        for key, val in payload.items():
+            assert getattr(infos, key) == val
+
+    @pytest.mark.parametrize(
+        "user_maker", [subscriber_user.make, lambda: baker.make(User)]
+    )
+    @pytest.mark.django_db
+    def test_create_infos(self, client: Client, user_maker, payload):
+        user = user_maker()
+        client.force_login(user)
+        assert not BillingInfo.objects.filter(customer__user=user).exists()
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == 200
+        user.refresh_from_db()
+        assert hasattr(user, "customer")
+        infos = BillingInfo.objects.get(customer__user=user)
+        assert hasattr(user.customer, "billing_infos")
+        assert infos.customer == user.customer
+        for key, val in payload.items():
+            assert getattr(infos, key) == val
+
+    def test_invalid_data(self, client: Client, payload: dict[str, str]):
+        user = subscriber_user.make()
+        client.force_login(user)
+        # address_1, zip_code and country are missing
+        del payload["city"]
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == 422
+        user.customer.refresh_from_db()
+        assert not hasattr(user.customer, "billing_infos")
+
+    @pytest.mark.parametrize(
+        ("operator_maker", "expected_code"),
+        [
+            (subscriber_user.make, 403),
+            (lambda: baker.make(User), 403),
+            (lambda: baker.make(User, is_superuser=True), 200),
+        ],
+    )
+    def test_edit_other_user(
+        self, client: Client, operator_maker, expected_code: int, payload: dict
+    ):
+        user = subscriber_user.make()
+        client.force_login(operator_maker())
+        baker.make(BillingInfo, customer=user.customer)
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == expected_code
+
+    @pytest.mark.parametrize(
+        "phone_number",
+        ["+33612345678", "0612345678", "06 12 34 56 78", "06-12-34-56-78"],
+    )
+    def test_phone_number_format(
+        self, client: Client, payload: dict, phone_number: str
+    ):
+        """Test that various formats of phone numbers are accepted."""
+        user = subscriber_user.make()
+        client.force_login(user)
+        payload["phone_number"] = phone_number
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == 200
+        infos = BillingInfo.objects.get(customer__user=user)
+        assert infos.phone_number == "0612345678"
+        assert infos.phone_number.country_code == 33
+
+    def test_foreign_phone_number(self, client: Client, payload: dict):
+        """Test that a foreign phone number is accepted."""
+        user = subscriber_user.make()
+        client.force_login(user)
+        payload["phone_number"] = "+49612345678"
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == 200
+        infos = BillingInfo.objects.get(customer__user=user)
+        assert infos.phone_number.as_national == "06123 45678"
+        assert infos.phone_number.country_code == 49
+
+    @pytest.mark.parametrize(
+        "phone_number", ["061234567a", "06 12 34 56", "061234567879", "azertyuiop"]
+    )
+    def test_invalid_phone_number(
+        self, client: Client, payload: dict, phone_number: str
+    ):
+        """Test that invalid phone numbers are rejected."""
+        user = subscriber_user.make()
+        client.force_login(user)
+        payload["phone_number"] = phone_number
+        response = client.put(
+            reverse("api:put_billing_info", args=[user.id]),
+            json.dumps(payload),
+            content_type="application/json",
+        )
+        assert response.status_code == 422
+        assert not BillingInfo.objects.filter(customer__user=user).exists()
+
+
+class TestStudentCard(TestCase):
+    """Tests for adding and deleting Stundent Cards
+    Test that an user can be found with it's student card.
+    """
+
+    @classmethod
+    def setUpTestData(cls):
+        cls.krophil = User.objects.get(username="krophil")
+        cls.sli = User.objects.get(username="sli")
+        cls.skia = User.objects.get(username="skia")
+        cls.root = User.objects.get(username="root")
+
+        cls.counter = Counter.objects.get(id=2)
+
+    def setUp(self):
+        # Auto login on counter
+        self.client.post(
+            reverse("counter:login", args=[self.counter.id]),
+            {"username": "krophil", "password": "plop"},
+        )
+
+    def test_search_user_with_student_card(self):
+        response = self.client.post(
+            reverse("counter:details", args=[self.counter.id]),
+            {"code": "9A89B82018B0A0"},
+        )
+
+        assert response.url == reverse(
+            "counter:click",
+            kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+        )
+
+    def test_add_student_card_from_counter(self):
+        # Test card with mixed letters and numbers
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "8B90734A802A8F", "action": "add_student_card"},
+        )
+        self.assertContains(response, text="8B90734A802A8F")
+
+        # Test card with only numbers
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "04786547890123", "action": "add_student_card"},
+        )
+        self.assertContains(response, text="04786547890123")
+
+        # Test card with only letters
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "ABCAAAFAAFAAAB", "action": "add_student_card"},
+        )
+        self.assertContains(response, text="ABCAAAFAAFAAAB")
+
+    def test_add_student_card_from_counter_fail(self):
+        # UID too short
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "8B90734A802A8", "action": "add_student_card"},
+        )
+        self.assertContains(
+            response, text="Ce n'est pas un UID de carte étudiante valide"
+        )
+
+        # UID too long
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "8B90734A802A8FA", "action": "add_student_card"},
+        )
+        self.assertContains(
+            response, text="Ce n'est pas un UID de carte étudiante valide"
+        )
+
+        # Test with already existing card
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "9A89B82018B0A0", "action": "add_student_card"},
+        )
+        self.assertContains(
+            response, text="Ce n'est pas un UID de carte étudiante valide"
+        )
+
+        # Test with lowercase
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "8b90734a802a9f", "action": "add_student_card"},
+        )
+        self.assertContains(
+            response, text="Ce n'est pas un UID de carte étudiante valide"
+        )
+
+        # Test with white spaces
+        response = self.client.post(
+            reverse(
+                "counter:click",
+                kwargs={"counter_id": self.counter.id, "user_id": self.sli.id},
+            ),
+            {"student_card_uid": "              ", "action": "add_student_card"},
+        )
+        self.assertContains(
+            response, text="Ce n'est pas un UID de carte étudiante valide"
+        )
+
+    def test_delete_student_card_with_owner(self):
+        self.client.force_login(self.sli)
+        self.client.post(
+            reverse(
+                "counter:delete_student_card",
+                kwargs={
+                    "customer_id": self.sli.customer.pk,
+                    "card_id": self.sli.customer.student_cards.first().id,
+                },
+            )
+        )
+        assert not self.sli.customer.student_cards.exists()
+
+    def test_delete_student_card_with_board_member(self):
+        self.client.force_login(self.skia)
+        self.client.post(
+            reverse(
+                "counter:delete_student_card",
+                kwargs={
+                    "customer_id": self.sli.customer.pk,
+                    "card_id": self.sli.customer.student_cards.first().id,
+                },
+            )
+        )
+        assert not self.sli.customer.student_cards.exists()
+
+    def test_delete_student_card_with_root(self):
+        self.client.force_login(self.root)
+        self.client.post(
+            reverse(
+                "counter:delete_student_card",
+                kwargs={
+                    "customer_id": self.sli.customer.pk,
+                    "card_id": self.sli.customer.student_cards.first().id,
+                },
+            )
+        )
+        assert not self.sli.customer.student_cards.exists()
+
+    def test_delete_student_card_fail(self):
+        self.client.force_login(self.krophil)
+        response = self.client.post(
+            reverse(
+                "counter:delete_student_card",
+                kwargs={
+                    "customer_id": self.sli.customer.pk,
+                    "card_id": self.sli.customer.student_cards.first().id,
+                },
+            )
+        )
+        assert response.status_code == 403
+        assert self.sli.customer.student_cards.exists()
+
+    def test_add_student_card_from_user_preferences(self):
+        # Test with owner of the card
+        self.client.force_login(self.sli)
+        self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8F"},
+        )
+
+        response = self.client.get(
+            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
+        )
+        self.assertContains(response, text="8B90734A802A8F")
+
+        # Test with board member
+        self.client.force_login(self.skia)
+        self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8A"},
+        )
+
+        response = self.client.get(
+            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
+        )
+        self.assertContains(response, text="8B90734A802A8A")
+
+        # Test card with only numbers
+        self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "04786547890123"},
+        )
+        response = self.client.get(
+            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
+        )
+        self.assertContains(response, text="04786547890123")
+
+        # Test card with only letters
+        self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "ABCAAAFAAFAAAB"},
+        )
+        response = self.client.get(
+            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
+        )
+        self.assertContains(response, text="ABCAAAFAAFAAAB")
+
+        # Test with root
+        self.client.force_login(self.root)
+        self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8B"},
+        )
+
+        response = self.client.get(
+            reverse("core:user_prefs", kwargs={"user_id": self.sli.id})
+        )
+        self.assertContains(response, text="8B90734A802A8B")
+
+    def test_add_student_card_from_user_preferences_fail(self):
+        self.client.force_login(self.sli)
+        # UID too short
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8"},
+        )
+
+        self.assertContains(response, text="Cet UID est invalide")
+
+        # UID too long
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8FA"},
+        )
+        self.assertContains(response, text="Cet UID est invalide")
+
+        # Test with already existing card
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "9A89B82018B0A0"},
+        )
+        self.assertContains(
+            response, text="Un objet Student card avec ce champ Uid existe déjà."
+        )
+
+        # Test with lowercase
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8b90734a802a9f"},
+        )
+        self.assertContains(response, text="Cet UID est invalide")
+
+        # Test with white spaces
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": " " * 14},
+        )
+        self.assertContains(response, text="Cet UID est invalide")
+
+        # Test with unauthorized user
+        self.client.force_login(self.krophil)
+        response = self.client.post(
+            reverse(
+                "counter:add_student_card", kwargs={"customer_id": self.sli.customer.pk}
+            ),
+            {"uid": "8B90734A802A8F"},
+        )
+        assert response.status_code == 403
+
+
+class TestCustomerAccountId(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.user_a = User.objects.create(
+            username="a", password="plop", email="a.a@a.fr"
+        )
+        user_b = User.objects.create(username="b", password="plop", email="b.b@b.fr")
+        user_c = User.objects.create(username="c", password="plop", email="c.c@c.fr")
+        Customer.objects.create(user=cls.user_a, amount=10, account_id="1111a")
+        Customer.objects.create(user=user_b, amount=0, account_id="9999z")
+        Customer.objects.create(user=user_c, amount=0, account_id="12345f")
+
+    def test_create_customer(self):
+        user_d = User.objects.create(username="d", password="plop")
+        customer, created = Customer.get_or_create(user_d)
+        account_id = customer.account_id
+        number = account_id[:-1]
+        assert created is True
+        assert number == "12346"
+        assert len(account_id) == 6
+        assert account_id[-1] in string.ascii_lowercase
+        assert customer.amount == 0
+
+    def test_get_existing_account(self):
+        account, created = Customer.get_or_create(self.user_a)
+        assert created is False
+        assert account.account_id == "1111a"
+        assert account.amount == 10
+
+
+@pytest.mark.django_db
+def test_update_balance():
+    customers = baker.make(Customer, _quantity=5, _bulk_create=True)
+    refills = [
+        *refill_recipe.prepare(
+            customer=iter(customers),
+            amount=iter([30, 30, 40, 50, 50]),
+            _quantity=len(customers),
+            _save_related=True,
+        ),
+        refill_recipe.prepare(customer=customers[0], amount=30, _save_related=True),
+        refill_recipe.prepare(customer=customers[4], amount=10, _save_related=True),
+    ]
+    Refilling.objects.bulk_create(refills)
+    sales = [
+        *sale_recipe.prepare(
+            customer=iter(customers),
+            _quantity=len(customers),
+            unit_price=10,
+            quantity=1,
+            _save_related=True,
+        ),
+        *sale_recipe.prepare(
+            customer=iter(customers[:3]),
+            _quantity=3,
+            unit_price=5,
+            quantity=2,
+            _save_related=True,
+        ),
+        sale_recipe.prepare(
+            customer=customers[4], quantity=1, unit_price=50, _save_related=True
+        ),
+    ]
+    Selling.objects.bulk_create(sales)
+    # customer 0 = 40, customer 1 = 10€, customer 2 = 20€,
+    # customer 3 = 40€, customer 4 = 0€
+    customers_qs = Customer.objects.filter(pk__in={c.pk for c in customers})
+    # put everything at zero to be sure the amounts were wrong beforehand
+    customers_qs.update(amount=0)
+    customers_qs.update_amount()
+    for customer, amount in zip(customers, [40, 10, 20, 40, 0]):
+        customer.refresh_from_db()
+        assert customer.amount == amount
diff --git a/rootplace/views.py b/rootplace/views.py
index 4aefb8c3..60204634 100644
--- a/rootplace/views.py
+++ b/rootplace/views.py
@@ -123,7 +123,7 @@ def merge_users(u1: User, u2: User) -> User:
         c_dest, created = Customer.get_or_create(u1)
         c_src.refillings.update(customer=c_dest)
         c_src.buyings.update(customer=c_dest)
-        c_dest.recompute_amount()
+        Customer.objects.filter(pk=c_dest.pk).update_amount()
         if created:
             # swap the account numbers, so that the user keep
             # the id he is accustomed to