Allow sales-channels to add information to invoice settings

Yes, this is oddly specific, but we spent days finding another way out
to make sense of this toggle since neither True nor False are truly
useful. Now, let's recommend False and allow posbackend to overrule it.

src/pretix/base/channels.py

@@ -22,6 +22,7 @@
 import logging
 import warnings
 from collections import OrderedDict
+from typing import Optional
 
 from django.dispatch import receiver
 from django.utils.translation import gettext_lazy as _
@@ -124,12 +125,20 @@ class SalesChannelType:
         return True
 
     @property
-    def required_event_plugin(self) -> str:
+    def required_event_plugin(self) -> Optional[str]:
         """
         Name of an event plugin that is required for this sales channel to be useful. Defaults to ``None``.
         """
         return
 
+    @property
+    def invoice_setting_suffix(self) -> Optional[str]:
+        """
+        Label for the sales channel selection in invoicing settings to add a e.g. a "recommended" or
+        "not recommended" flag.
+        """
+        return
+
 
 def get_all_sales_channel_types():
     from pretix.base.signals import register_sales_channel_types

src/pretix/control/forms/event.py

@@ -46,7 +46,7 @@ from django.db.models import Prefetch, Q, prefetch_related_objects
 from django.forms import formset_factory, inlineformset_factory
 from django.urls import reverse
 from django.utils.functional import cached_property, lazy
-from django.utils.html import escape, format_html
+from django.utils.html import conditional_escape, escape, format_html
 from django.utils.safestring import mark_safe
 from django.utils.timezone import get_current_timezone_name
 from django.utils.translation import gettext, gettext_lazy as _, pgettext_lazy
@@ -1091,7 +1091,7 @@ class InvoiceSettingsForm(EventSettingsValidationMixin, SettingsForm):
     ]
 
     invoice_generate_sales_channels = forms.MultipleChoiceField(
-        label=_('Generate invoices for Sales channels'),
+        label=_('Sales channels to generate invoices for'),
         choices=[],
         widget=forms.CheckboxSelectMultiple,
         help_text=_("If you have enabled invoice generation in the previous setting, you can limit it here to specific "
@@ -1121,9 +1121,17 @@ class InvoiceSettingsForm(EventSettingsValidationMixin, SettingsForm):
             self.fields['invoice_numbers_prefix_cancellations'].widget.attrs['placeholder'] = event.slug.upper() + '-'
         locale_names = dict(settings.LANGUAGES)
         self.fields['invoice_language'].choices = [('__user__', _('The user\'s language'))] + [(a, locale_names[a]) for a in event.settings.locales]
-        self.fields['invoice_generate_sales_channels'].choices = (
-            (c.identifier, c.label) for c in event.organizer.sales_channels.all()
-        )
+        self.fields['invoice_generate_sales_channels'].choices = [
+            (
+                c.identifier,
+                mark_safe(
+                    '{} {}'.format(
+                        conditional_escape(c.label),
+                        conditional_escape(c.type_instance.invoice_setting_suffix or '')
+                    ).strip()
+                )
+            ) for c in event.organizer.sales_channels.all()
+        ]
         pps = [str(pp.verbose_name) for pp in event.get_payment_providers().values() if pp.requires_invoice_immediately]
         if pps:
             generate_paid_help_text = _('An invoice will be issued before payment if the customer selects one of the following payment methods: {list}').format(

src/pretix/locale/de/LC_MESSAGES/django.po

@@ -14478,8 +14478,8 @@ msgstr ""
 "erreichbar sind."
 
 #: pretix/control/forms/event.py
-msgid "Generate invoices for Sales channels"
-msgstr "Rechnungen nur für bestimmte Verkaufskanäle generieren"
+msgid "Sales channels to generate invoices for"
+msgstr "Verkaufskanäle mit Rechnungsgenerierung"
 
 #: pretix/control/forms/event.py
 msgid ""

src/pretix/locale/de_Informal/LC_MESSAGES/django.po

@@ -10562,8 +10562,8 @@ msgstr ""
 "angegebenen „letzten Zahlungsdatum“ akzeptiert."
 
 #: pretix/base/settings.py
-msgid "Allow postponed payment choice for sales channels"
-msgstr "Erlaube aufgeschobene Entscheidung der Zahlungsart für Verkaufskanäle"
+msgid "Sales channels to generate invoices for"
+msgstr "Verkaufskanäle mit Rechnungsgenerierung"
 
 #: pretix/base/settings.py
 msgid ""