Shipping via USPS except some items need to be flat rate

Hello, I was wanting to know how (if possible) to ship only specific items as flat rate, and everything else defaults to USPS.

Currently I'm using USPS shipping as my only option.

You could do this by customizing the usps.php shipping module ...

Is there anything other than products_id to identify these Products in the products table such as their master_categories_id or their manufacturers_id that can be used to identify them?

The idea is to set their weight to 0 and then calculate how many are in the cart and add the flat rate charge to the USPS shipping total ...

1 if you buy 1 or 3 of Product A that is one of these Flat Rate products is it $XX.XX * 3 or just $XX.XX

2 what happens if there are only these Flat Rate Products in the cart, should the minimal weight quote from USPS be used on the various shipping methods plus the Flat Rate charge ... or just one method be posted