My copy of Adventure Path number 51 was waiting for me in the mailbox today ... and to my dismay the envelope appeared to be in quite a bad state – three of the corners were bent, there was a big crease down the middle of the envelope where it appeared someone had tried to bend or fold it, and the whole thing was kind of warped.
I dreaded to think what the book inside would look like ... but when I opened it, the book was in almost perfect condition, with just a very slight bend to one of the corners of the front cover. Not bad for a soft-cover book inside a pretty badly mangled envelope!
So this is a long way of saying good work on your choice of envelopes Paizo!