Yeah, they've been cracking down on media mail lately... The guys at my PO said they were told it could only be educational media.... Here's what it says on the usps site:
Media Mail® service is a cost efficient way to mail books, sound recordings, recorded video tapes, printed music, and recorded computer-readable media (such as CDs, DVDs, and diskettes). Media Mail can not contain advertising except for incidental announcements of books. The maximum weight for Media Mail is 70 lbs.
There are presorted rates available for bulk quantities of Media Mail (minimum quantity is 300 pieces).
A barcode discount is available for Media Mail.
Rates are based on weight and size.