For all who are interested, the clear envelopes I use to mail my fabric postcards come from here. The one I use is # B 46 It is 6 1/4" x 4 1/2" with the opening on the 6" side. They have a peel and stick re- sealable flap. Remember to use the self stick stamps. I normally take mine to the PO and let them weigh and stamp them. Sometimes they weigh a little more and need extra postage. Also if your cards are more than 1/8 th inch thick they cost extra. Usually though, most of mine go thru for the cost of a first class stamp. I always get them to hand cancel them, so they don't go thru the machine . I had an issue of 1 getting totally mangled once going thru the machine. No problems since using the envelopes and getting them hand cancelled.
Some others that I know just send them thru the system with no envelope and no other special care and have done fine. I can only go by my experience and it has shown me that I prefer the envelopes and hand cancelling.