This sounds like you have left a backup copy in the directory:
/includes/modules/shipping
and that is a auto-loading directory ... so this will produce errors if you have additional renamed copies of usps.php as they will conflict ...
Check to ensure that you do not ...
Another cause could also be if you have the language file in the code directory or visa versa ...
Another cause could be a bad upload of the new files ...
Meanwhile, ensure that you have the current copy of USPS being installed:
http://www.zen-cart.com/showthread.php?215103-USPS-RateV4-Intl-RateV2-October-30-2014-K6-for-November-2-2014
This is also a good time to subscribe to the News & Announcements forum to keep updated on changes that may or will affect you:
http://www.zen-cart.com/forumdisplay.php?2-Zen-Cart-Release-Announcements