You can not use priority supplies to package your shipments that are not priority in any way.  Today I received one in a priority box and then in a priority mail.  If that had been discovered by the USPS, I would have been charged postage due for the difference in price.  Media Mail is subject to inspection.  They randomly select packages and open them to make sure they qualify for those rates.  However, it could get discovered with other rates too.  If packaging the outer packaging gets damaged and it shows the Priority packaging that package will be marked up postage due.

This has been announced here over and over again for years......I just can't believe there are still members who are cheating this way and risking PBS losing their ability to mail our books using media mail rates. (at least that's what we've been told before)