If you look in the file:
/includes/modules/shipping/usps.php

you will find around the line 214 the line:
Code:
    $this->_setFirstClassType('FLAT');
If you change that to read:
Code:
//    $this->_setFirstClassType('FLAT');
    $this->_setFirstClassType('LETTER');
//    $this->_setFirstClassType('PARCEL');
What that does is change First Class to give LETTER quotes ... the other lines are commented out so that if you want to test the results of the other settings you can ...