Google Checkout shopping cart has made a few improvements in other systems I’ve seen. Adding a cart bring up a shopping cart in the upper right corner. Changing the quantity automatically updates the shopping cart information. If you do not view the cart for a time, it will automatically minimize.
Official Google Checkout Blog: Button up your website with the Google Checkout shopping cart: “With the Google Checkout shopping cart, your buyers can add multiple items to their shopping cart, change item quantities, and remove items — all without leaving your site.”