Enhance payment method validation

src/pretix/plugins/banktransfer/payment.py

@@ -26,3 +26,6 @@ class BankTransfer(BasePaymentProvider):
 
     def checkout_prepare(self, request, total):
         return True
+
+    def checkout_is_valid_session(self, request):
+        return True

src/pretix/plugins/stripe/payment.py

@@ -17,3 +17,6 @@ class Stripe(BasePaymentProvider):
              required=False
          ))
     ])
+
+    def checkout_is_valid_session(self, request):
+        return False