Show message and cart after tax rate has changed

src/pretix/base/services/cart.py

@@ -1063,6 +1063,7 @@ def update_tax_rates(event: Event, cart_id: str, invoice_address: InvoiceAddress
         if pos.tax_rate != rate:
             current_net = pos.price - pos.tax_value
             new_gross = pos.item.tax(current_net, base_price_is='net', invoice_address=invoice_address).gross
+            totaldiff += new_gross - pos.price
             pos.price = new_gross
             pos.includes_tax = rate != Decimal('0.00')
             pos.override_tax_rate = rate

src/pretix/presale/checkoutflow.py

@@ -554,11 +554,16 @@ class QuestionsStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
             addr = self.invoice_form.save()
             self.cart_session['invoice_address'] = addr.pk
 
-            update_tax_rates(
+            diff = update_tax_rates(
                 event=request.event,
                 cart_id=get_or_create_cart_id(request),
                 invoice_address=self.invoice_form.instance
             )
+            if abs(diff) > Decimal('0.001'):
+                messages.info(request, _('Due to thec invoice address you entered, we need to apply a different tax '
+                                         'rate to your purchase and the price of the products in your cart has '
+                                         'changed accordingly.'))
+                return redirect(self.get_next_url(request) + '?open_cart=true')
 
         return redirect(self.get_next_url(request))
 

src/pretix/presale/templates/pretixpresale/event/checkout_base.html

@@ -3,7 +3,7 @@
 {% load bootstrap3 %}
 {% block title %}{% trans "Checkout" %}{% endblock %}
 {% block content %}
-    <details class="panel panel-default cart">
+    <details class="panel panel-default cart" {% if "open_cart" in request.GET %}open{% endif %}>
         <summary class="panel-heading">
             <h3 class="panel-title">
                 <span>