The new USPS January 22, 2012 release is in the Free Software Add Ons at:
http://www.zen-cart.com/index.php?ma...oducts_id=1962

Makes sure that you followed all the instructions in the readme.txt file ...

You can also test if you installed it correctly by doing a search in the Tools ... Developers Tool Kit ... in the bottom input box enter:
usps.php

and you should see the version id for:
* @version $Id: usps.php 18348F 2012-01-22 01:22:00Z ajeh $

Then, check if you are using Tare Rates in the Configuration ... Shipping/Packaging ... if so, are they making the weight too high for First Class? Or, do you need to change the decimal places from 3 to 2 on the USPS configuration? Note: in some cases you might need to change this to 1 ...