I just ordered from a company that folded a total of 7 yards and placed it in a flat-rate envelope ($4.95)...and, when you and I traded fabrics it was a similar amount, which cost like $7-$8 (can't remember) in a polymailer. How much did it weigh? Geez, I dunno! :D
FWIW, the rule on flat rate envelopes is you may reinforce the closure with a single length of tape, only on the back side. As long as you don't cut the seams to make the envelope bigger, you can cram as much as you want in there!