update totals

cart-grain.go

@@ -285,7 +285,7 @@ func (c *CartGrain) HandleMessage(message *Message, isReplay bool) (*FrameWithPa
 			existingItem, found := c.FindItemWithSku(msg.Sku)
 			if found {
 				existingItem.Quantity += int(msg.Quantity)
-				c.TotalPrice += existingItem.Price * int64(msg.Quantity)
+				c.UpdateTotals()
 			} else {
 				return c.AddItem(msg.Sku, int(msg.Quantity)) // extent AddRequest to include quantity
 			}