There is a lot going on when Magento actually sends a cart to Google Checkout, even more so if you are using Merchant calculated shipping. As a result lots of things can go wrong, with the merchant calculated shipping callback or with the actual server to server posting of the cart contents.
To help you with diagnosing problems I thought I’d shared my top 3 methods for finding out what is going wrong when Magento and Google Checkout are not communicating properly.
1) The Intergration Console
The first and most obvious place to try is the Google Checkout Intergration Console. As you can see in the screenshot below the console is hidden away in the Tools tab of the main seller dashboard. You’ll find it at the bottom of the left menu.
If the Google callbacks have failed or incorrect XML has been sent to Google you will find a report of the errors here. It is a good idea to keep an eye on the timestamps as you do not want to wind up chasing an old problem. It’s also good to periodically check this console while in production, just in case something starts to go wrong. I found when I upgraded to the latest Magento that the HTTPS callback was failing, but I hadn’t found it during development because I do not use SSL in development environments.