I use the Manilla envelopes, but I remove the metal clasp in the back of the envelope (I have found that it catches on the mail sorters and causes the envelopes the tear.
First I plastic bag the fabric before putting in the Manilla envelope - & use packing tape (I got some a while ago that was thinner than the normal packing tape - LOVE IT - sticks better)- tape all sides & center of envelope.
If I use a Flat Rate envelope I tape all 3 sides before packing & closing than criss cross the envelope with more tape. Fabric usually goes in several plastic bags.
HANDLE WITH CARE - written in large lettering over back and front of envelope