I both upgraded from zencart 1.3.0.1 to 1.3.6 and I added the google checkout to my site and zencart. However, I think this problem has more to do with the upgrade because I'm having a problem with the create_account.php page (and the google checkout code doesn't have this page). The user selects a country from the drop down list. The $POST array contains a 'country' item but the code is looking for a 'zone_country_id' item so it never sees the country and does not let the user create an account.
Has anyone else encountered this problem? I had to customize zencart since I sell mobile software and the user had to select a carrier from a drop down when creating a new account and this has compilcated upgrading.
If anyone has an understanding of how the zones work or any clues as to this problem, please let me know.
This was a bug in version 1.3.5 - have you tried removing your customized "create account" templates and seeing if the error persists ? If it solves with the removal of that page then you know that it is an error in the template and not the cart itself. Which I might add was the problem many times during the "bug" discovery ;)