I use the regular "legal" sized envelopes. I fold my fabric flat and wrap with the press n seal. I put an addy label either in or on that so that if it gets lost, it comes back to me. I also put in a letter for the swapper to know my wishes for colors, etc.
Since I used the Dr. Seuss fabrics this time (they were thicker) it was 65 cents for me to mail them. Last time it cost me 45 cents. My best suggestion is to take a couple to the post office, have them weigh them and then buy your postage, stamp 'em there and send 'em right out.
Good luck!!!