Old photo paper boxes also work great.

I think this is an iffy proposition. I sent a submission to a magazine in England and made the mistake to send the prints in a photo paper box. The sent me an e mail saying the box had arrived bent and had creased some of the prints. Fortunatelly some of the prints were fine and they published 3.

I have come to the conclusion that the mail in any country acts as collaborators so that if anything can be done to the package it will be done, either leaving or delivering.... :-)