Media mail is subject to random searches, due to its low cost rate. Book rate is another one with that restriction, also due to cost.
The seller from Amazon.com is sending that stuff media mail, and by the restrictions of media mail, he shouldn't have sent it that way.
Media mail shippable items:
[quote name='From USPS website']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.[/quote]
There's nothing in the 12-month XBL kit or a 12-month XBL card that would be shippable via media mail. If it was a game, then it would be OK to ship media mail, as it's computer-readable media.
I'm sure his local PO has seen hundreds of media mail shipments coming from him, and probably picked a few random ones to open and check for contents.