There are two small priority boxes that are close in size but not exactly the same. The smaller one is flat rate. The other one you pay by weight and distance.
It is illegal to use Priority mailing supplies for anything other than Priority even if you wrap it, turn it inside out, or reuse it. DON'T DO IT.
You can order all Priority mailers free at USPS.com. You can use your own box for Priority as well. They can put a Priority sticker on it at the PO.