I've updated some of Alex's code from his contribution to handle a few errors (some not of his own) in the processing. I've also corrected a query for it to function properly with CRE Loaded. There are also some other debugging messages included for now for development purposes. Most of the fixes I made are captured in the comments within the GoogleCheckout.php file at the top.
http://stenod.derty.org/googlecheckout.php