If you want it sent back priority then then it will fit into a flat rate box, I got one package today and it was sent priority and cost a little more then $8. What I used to do in a swap is I take it to the post office and have them tell me how much it'll cost to mail and then buy two or three extra stamps. if I didn't need them I get them back, but they are a safety in case what I get back weighs more then what I shipped. But now with flat rate envelopes and boxes for priority. I fill out a address label and flat rate stamp and stick it in the package that I'm mailing.