I don't know that it will solve all issues but it is a good place to start with the current USPS March 7, 2014 K3 version and then we can go from there ...

If it does not solve the issue, edit the USPS module and at the bottom, set the Debug to Logs and test a shipment for Canada then check you directory /cache (this would be /logs on Zen Cart v1.5.1+) and Zip the log file and post it as an attachment on your next post ...