My daughter works for the Post Office. I went to send a slipcase to one of our members here and asked her just that question--could it go Media Mail? She responded that Media Mail was for books and printed materials only and if it is just the slipcase, it didn't qualify. She said she could get in trouble if she okayed it. Now that I know this, the next time I ship a slipcase only, I'll also enclose a really cheap book so that it would qualify.
John