If you want a flat rate cost, ignore the USPS module, and use a flat rate method you can define. If you're already using the built-in Flat Rate Shipping, your options are:
1. Use Weight Based Shipping and set up multiple flat rates within it. You can do this by following my tutorial here:
http://www.opencartx.com/tutorials#multiple-flat-rates
2. Use a commercial extension like
Multi Flat Rate Shipping that can easily create (and name) different flat rates within its admin panel.