It depends on who you are working with at the PO. My mail clerk was very kind and weighed each of my 35 envelopes and not one (except for Canada) needed extra postage. She had a plastic template with an envelope size slit in it that she slipped an envelope through to see if it would fit OK. And, my envelopes did. I pressed all my fabric and folded it so that it would lay the flatest, least bulky that I could get it, then I squeezed as much air out as I could before sealing.
Since I am new to these swaps I will tell you how I am keeping track of the fabric that comes in: I open the envelope, and on the envelope I write a description of the fabric itself, the forum name of the owner, the day I received it and a check mark the date I recorded it here. I do all the recording on the master sheet, then I select fabric to coordinate with the F8 and slip it and the envelope etc into a plastic sleeve, that way everything from each member stays together. So far, so good.