The current USPS code reads:
Code:
          //PRIORITY MAIL OPTIONS
          if ($key == 'PRIORITY'){
            $this->container = ''; // Blank, Flate Rate Envelope, or Flat Rate Box // Sm Flat Rate Box, Md Flat Rate Box and Lg Flat Rate Box

          }
So you would need to change that to match your needs with the code posted ...

The Flat Rate flat shipping module is for when you want to charge 1 price regardless of the number of Products or Weight of the Products ...

I do not believe you want to use the Flat Rate flat shipping module at all and you should go to the Modules ... Shipping ... and click on it and click REMOVE ...