I just sent a piece of fabric - less than a FQ - and had a hard time getting it to fit in a #10 envelope - I pressed it and did the careful folding to make it fit - and did not enclose the card or plastic baggie - it did make it to it's destination okay, but I was fretting about it.
I try to find a balance between convenience and "reasonable" - I won't pay (or charge) for a FRE for one yard or less of fabric. A fairly accurate scale is very convenient to have around - it's also great for measuring food - ingredients or portions - if one only mails things once in a while.
I will only fill a FRE to "moderate distortion" - about six yards is all I can fairly easily get in the regular sized FRE. It also depends to a certain extent on how hefty the fabric is.
I usually also put the fabric with a "to and from" card in a plastic bag and that goes into the outside mailer - whether a manila envelope or a Priority Mail product.
The Priority Flat Rate containers (envelopes and boxes) ARE convenient - but not always the most economical way to send things. The USPS site (and a scale) is definitely helpful when trying to figure out the least expensive way to mail something.