i just shipped 4.5 yds + in a priority mail, flat rate $4.95 thing. i don't know if they call it an envelope. it's made of cardboard.
it started out flat, but believe me, it wasn't flat after i filled it. how could it be? you're allowed to put in up to 4 lbs. they didn't like the tape all over it, but they didn't have a problem with the bulge at all. and it was a recycled (re-used) one, too.