Received my batch from the ebay seller. They look a little thin to me, they are suppose to be protective, so I'm going to give them a try with tomorrow's mailings.

I tried to put my finger thru them and they didn't break/tear...so I guess they will do.

I got fabrics I bought on line in a USPS cardboard mailer. The mailer was torn open - I mean WIDE open. Fortunately, the fabric was still inside. There was no attempt by Post Office to reseal the mailer, which surprised me. In the past, mail that was damaged in transit has been sealed and a standard note of explanation was stamped on the envelope.

I think the poly mailers will work fine. They're very hard to open when you're trying, so not likely to open in transit.

i have used some for sending block they seem strong

Yeah, I think they will rank much higher than the cardboard/paper packaging...and of course, I'll still tape all the corners.