Media mail can be used to send *only* appropriate media (so don't mix your packages...one book in a box of stuff doesn't make the package media mail eligible) and what surprised me was that media that contains advertising is NOT eligible for media mail. This means you cannot send magazines via media mail (and if you add magazines to a package with books...no media mail!) which was news to me.
Sure enough the USPS website does state the no-advertising restriction:
[Content:] Generally used for books (at least eight pages), film (16 mm or narrower), printed music, printed test materials, video and sound recordings, playscripts, printed educational charts, loose-leaf pages and binders consisting of medical information, and computer-readable media. Sound recordings may include incidental announcements of recordings and guides or scripts prepared solely for use with such recordings. Books may contain no advertising other than incidental announcements of other books.[emphasis mine]
The media mail stamp now contains a notice that media mail is subject to inspection so they can enforce these restrictions. Which makes a certain amount of sense, but the bears are really out of control lately.