I thought Media Mail could only be used for certain items, usually educational.
No. See below.
I believe all books or guides with limited advertisement qualify. Other media such as DVD's, CD's, Vhs have to be educational. Anything else is prohibited but also depends on how strict your post office may be. I once accidentally shipped a teddy bear through media and he made it through ha.
The "educational" thing doesn't really mean anything. Who is the USPS to determine what book, DVD or CD is "educational" compared to another? Someone can watch something like Prometheus on DVD for entertainment, or they could watch it for an educational film theory class. The USPS has no idea. So it's either all or nothing, and they go with letting all items in those categories through because any other standard is impossible to enforce.
This is straight from the USPS website:
4.0 Content Standards for Media Mail 4.1 Qualified Items
Only these items may be mailed at the Media Mail prices:
a. Books, including books issued to supplement other books, of at least eight printed pages, consisting wholly of reading matter or scholarly bibliography, or reading matter with incidental blank spaces for notations and containing no advertising matter other than incidental announcements of books. Advertising includes paid advertising and the publishers' own advertising in display, classified, or editorial style.
b. 16-millimeter or narrower width films, which must be positive prints in final form for viewing, and catalogs of such films of 24 pages or more (at least 22 of which are printed). Films and film catalogs sent to or from commercial theaters do not qualify for the Media Mail price.
c. Printed music, whether in bound or sheet form.
d. Printed objective test materials and their accessories used by or on behalf of educational institutions to test ability, aptitude, achievement, interests, and other mental and personal qualities with or without answers, test scores, or identifying information recorded thereon in writing or by mark.
e. Sound recordings, including incidental announcements of recordings and guides or scripts prepared solely for use with such recordings. Video recordings and player piano rolls are classified as sound recordings.
f. Playscripts and manuscripts for books, periodicals, and music.
g. Printed educational reference charts designed to instruct or train individuals for improving or developing their capabilities. Each chart must be a single printed sheet of information designed for educational reference. The information on the chart, which may be printed on one or both sides of the sheet, must be conveyed primarily by graphs, diagrams, tables, or other nonnarrative matter. An educational reference chart is normally but not necessarily devoted to one subject. A chart on which the information is conveyed primarily by textual matter in a narrative form does not qualify as a printed educational reference chart for mailing at the Media Mail prices even if it includes graphs, diagrams, or tables. Examples of qualifying charts include maps produced primarily for educational reference, tables of mathematical or scientific equations, noun declensions or verb conjugations used in the study of languages, periodic table of elements, botanical or zoological tables, and other tables used in the study of science.
h. Loose-leaf pages and their binders consisting of medical information for distribution to doctors, hospitals, medical schools, and medical students.
i. Computer-readable media containing prerecorded information and guides or scripts prepared solely for use with such media.
In short, books 8 pages or more that contain only incidental advertising, like a page in the back saying "You can buy these books from the same great author!" That's why single issue comic books aren't supposed to ship via Media Mail, because of all of the advertising for random stuff in the pages. Printed material allowed also includes stuff like sheet music, educational testing material, plays, manuscripts, educational study charts, and medical binders using loose leaf paper.
You can also send movie film, but not film going to commercial movie theaters. Probably not a problem anymore since theaters have gone digital.
Sound recordings = CDs an the like, but also things like vinyl. For whatever reason, they class video recordings as sound recordings, so DVDs are fine too.
And finally, computer-readable media containing prerecorded information and guides or scripts indicates something like maybe book PDFs on a CD or something, but it doesn't mean general computer software or video games, hence why those are excluded from media mail.
So for the purposes of most CAGs, sending books, DVDs/Blu-rays and audio CDs via Medial Mail is perfectly fine and they don't have to be "educational." Video games have to be shipped via First Class Mail or Parcel Post unless you upgrade to something like Priority Mail.