In the function quote, you will see this line:
Code:
          $cost = preg_replace('/[^0-9.]/', '',  $cost);
That is converting the $cost to a value ...

Beneath that, before the array is build for the $methods, you can add additional code to raise this price so that it is now 1.2 times the cost received from USPS ...

Something like:
Code:
$cost = $cost * 1.2;
would raise ALL quotes from USPS by 20% ...