USPS works with RateV3 and RateV4 ...
You can get the current USPS
January 22, 2012 by reading the Announcement on this:
http://www.zen-cart.com/forum/showthread.php?t=191406
NOTE:you should also be upgrading to v1.3.9h or v1.5 ... due to all the bugs, security issues etc. with v1.3.7 ...