shipping option help

Hi all. I need help figuring out if what I want is possible.
I only want to offer USPS Flat rate priority shipping (since it is fast and has the tracking number).
I will charge $6 for the small box, $13 for medium, and $17 for Large boxes.

My main products (jewelry) can fit 4 to a small flat rate box. So I only want to charge $6 if they order 4 or less but if they order 5 or more or 1 jewelry plus larger items then charge them for a medium or large box.

I only want to charge the cost of shipping and not a per piece because if they order only 1 and I charge per item it will not cover the cost and I don't want to charge $6 per piece since that would be way too much if they ordered 3-4 items.

Is there a module out there that will know only to charge $6 up to 4 items but change the shipping method if they order 5 or more or 1 plus some other larger things.

You could do this by fudging in a weight for your smaller items and other weights for your larger items then use probably the table rate shipping module
Then set in your admin > config > shipping > max weight for a small box & max weight for a large box

Play with it until you get to where you want to be