I have read everything I can find on USPS flat rate shipping. I have gathered the following information from several different threads. I still cannot quite find a post that tells if/how it can be done.

I want to enable USPS Priority Flat Rate Shipping for most items. In fact, I think I only have 2 items that will not fit in a flat rate box. (Domestic only for now).

I went to /includes/modules/shipping/usps.php.

!. What line do I put it on?
2. Is this the correct php language?

global $shipping_weight;
//PRIORITY MAIL OPTIONS
if ($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

}
//PRIORITY MAIL OPTIONS
if ($key == 'PRIORITY')
{
if ($usps_shipping_weight < 1)
{
$this->container = 'Sm Flat Rate Box';
}
else if ($usps_shipping_weight < 5)
{
$this->container = 'Md Flat Rate Box';
}
else
{
$this->container = 'Lg Flat Rate Box';
}
}

Thanks so much.