I do not see USPS in your shipping choices ...

Make sure that you have in these files directories:
/includes/modules/shipping/usps.php
/includes/lanugages/english/modules/shipping/usps.php

If you do, do a search in the Tools ... Developers Tool Kit ... and in the bottom input box enter:
usps.php

and select Catalog and click search ...

What comes up on the filenames and the version id lines?