Could you please set your USPS Debug to Logs and try to obtain a quote via your checkout_shipping ...

The logs are found in either /cache or /logs depending on your Zen Cart version ...

Then Zip the log, they start with SHIP_usps and end in the Country 2 digit ISO code and Zip Code such as:
Domestic:
SHIP_usps_Debug_date_info_here_US_33601.log

International:
SHIP_usps_Debug_date_info_here_CA_M4B1B3.log

Once you Zip the log(s), could you then attach them to your next post, please?