You really need to run a current version of USPS ...

Look at the Known Bug Fixes for v1.3.8 ...
http://www.zen-cart.com/forum/showthread.php?t=82619

and at the Security Patches and Updates:
http://www.zen-cart.com/forum/showthread.php?t=131115

NOTE: upgrading to v1.3.9 from our Home page contain all of these and more ...