This is a valid concern. Autos, GU'd, thicker singles - should never be sent via PWE. I have a stack of cards that have all manner of bends, creases, dings that were sent PWE, and are now completely worthless.
PWE, when done properly, is an economical way to send smaller value trades. When done improperly, it is an easy way to have junk show up in the recipients mailbox. There is at least one thread on how to properly package cards - my big thing is that I package cards the way I would want to receive them. Throwing a forever stamp on an envelope and writing Do Not Bend and dropping it in your mailbox, is at best a 60/40 proposition that the cards will arrive undamaged.
Even taking a PWE to the post office, paying the 71 cents is no certainty of a PWE arriving safely. I opened one up this morning - sent via post office - the rigid card holder was smashed, fortunately, the cards don't appear to be damaged. Adding something as simple as a small cardboard backer and/or at least securing the cards in the envelope so they don't slide around would help eliminate a bunch of these issues.
