As Chainman recommended to me, the more selections you have the long it takes them to ping your site, the more likely it will time out.
I just realized that since I moved my shop directory up to the top i had to tell Google where to find the response handler. No wonder my invoices were not going through. DOH!
Prices and time for Parcel Post are pretty much antiquated by Priority.
Do you realize that it only costs $5-13 per week for UPS daily pickup? Total control of tracking. Pickup on demand is @ $5 per package.
Try sending yourself a GC invoice and see what you get in the integration panel for errors if any.
Check file permissions:
/googlecheckout/logs/response_error.log file. Logs are not writable, set them to 777