Not saying I haven't done it before but this is from the USPS:
Media Mail shipping is a cost-effective way to send educational materials. This service has restrictions on the type of media that can be shipped. Media Mail rates are limited to the items listed below:
Books (at least 8 pages).
Sound recordings and video recordings, such as CDs and DVDs.
Play scripts and manuscripts for books, periodicals, and music.
Computer-readable media containing prerecorded information and guides or scripts prepared solely for use with such media.
Sixteen millimeter or narrower width films.
Printed objective test materials and their accessories.
Printed educational reference charts.
Loose-leaf pages and their binders consisting of medical information for distribution to doctors, hospitals, medical schools, and medical students.
Media - printed music, sound recordings, periodicals, CDs, DVDs, and books.
Media Mail packages may not contain advertising. Comic books do not meet this standard.