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.