Actually this particualr issue may be solved easier than you expect, should this work ...

Go to:
http://www.zen-cart.com/downloads.php?do=file&id=1292

and download a fresh copy of USPS July 28, 2013 J5 ...

Now be sure to install the files in:
/includes/modules/shipping/usps.php
/includes/languages/english/modules/shipping/usps.php

Note: again the same test in the Developers Tool Kit will reveal that those two lines should read:
* @version $Id: usps.php 2013-07-30 ajeh Version J5 $

Then do the REMOVE, INSTALL and re-configure of the USPS shipping module in the Modules ... Shipping ...