I would recommend upgrading to v1.3.9a ...

But, if that is not an option, you could install the January 4, 2010 USPS update at:
http://www.zen-cart.com/forum/showthread.php?t=144992

Be sure to read the whole thread ...

You probably need all the Security Patches and Updates (note: these are also in v1.3.9a) at:
http://www.zen-cart.com/forum/showthread.php?t=131115

And you probably need all of the Known Bug Fixes for v1.3.8 (also in the v1.3.9a) at:
http://www.zen-cart.com/forum/showthread.php?t=82619

NOTE: if you are sure that you have the current USPS shipping module, be sure that you email or call USPS to activate your account for the Production Server ...