Fix shipping weight calculation

src/storefront/cart.py

@@ -153,10 +153,13 @@ class Cart:
     def get_total_price(self):
         return sum(self.get_item_prices())
 
+    def get_weight_for_all_items(self):
+        for item in self:
+            yield round(Decimal(item['variant'].weight.value) * item['quantity'], 3)
+
     def get_total_weight(self):
         if len(self) > 0:
-            for item in self:
-                return item['variant'].weight.value * sum(self.get_all_item_quantities())
+            return sum(self.get_weight_for_all_items())
         else:
             return 0