If you've used the Google Checkout Buy Now button, you know that it's a bit limiting - only allowing customers to purchase one item at a time. Google Checkout's new shopping cart allows your customers to purchase multiple items at once.
To get started, go to the Tools tab in the Google Checkout Merchant Center. Enter your product info, price and image location. An HTML script will be generated for you. Copy and paste the code onto your product pages. Test the button to make sure it's working and you're good to go.
For more advanced options, check out (no pun intended) the developer guide.