make Refilling.payment_method a SmallIntegerField

eboutic/models.py

@@ -253,7 +253,7 @@ class Invoice(models.Model):
                     customer=customer,
                     operator=self.user,
                     amount=i.product_unit_price * i.quantity,
-                    payment_method="CARD",
+                    payment_method=Refilling.PaymentMethod.CARD,
                     date=self.date,
                 )
                 new.save()