Widget: Pass cart IDs around in some more places

src/pretix/presale/views/cart.py

@@ -47,6 +47,9 @@ class CartActionMixin:
             u += '&require_cookie=true'
         else:
             u += '?require_cookie=true'
+        if 'iframe' in self.request.GET:
+            cart_id = get_or_create_cart_id(self.request)
+            u += '&cart_id={}'.format(cart_id)
         return u
 
     def get_success_url(self, value=None):