Add "is_implicit" attribute to payment providers

src/pretix/base/payment.py

@@ -50,6 +50,16 @@ class BasePaymentProvider:
     def __str__(self):
         return self.identifier
 
+    @property
+    def is_implicit(self) -> bool:
+        """
+        Returns whether or whether not this payment provider is an "implicit" payment provider that will
+        *always* and unconditionally be used if is_allowed() returns True and does not require any input.
+        This is  intended to be used by the FreePaymentProvider, which skips the payment choice page.
+        By default, this returns ``False``. Please do not set this if you don't know exactly what you are doing.
+        """
+        return False
+
     @property
     def is_meta(self) -> bool:
         """
@@ -552,6 +562,10 @@ class PaymentException(Exception):
 
 class FreeOrderProvider(BasePaymentProvider):
 
+    @property
+    def is_implicit(self) -> bool:
+        return True
+
     @property
     def is_enabled(self) -> bool:
         return True

src/pretix/presale/checkoutflow.py

@@ -487,9 +487,13 @@ class PaymentStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
 
     def is_applicable(self, request):
         self.request = request
-        if self._total_order_value == 0:
-            self.cart_session['payment'] = 'free'
-            return False
+
+        for p in self.request.event.get_payment_providers().values():
+            if p.is_implicit:
+                if p.is_allowed(request):
+                    self.cart_session['payment'] = p.identifier
+                    return False
+
         return True