I had this discussion with USPS a few times.
One clerk was “holier than thou” demanding nothing in a media mail package but a book (or media item).
I called the toll-free customer service and they had me talk to consumer affairs.
According to USPS and the DMM (domestic mail manual), you ARE allowed to include a packing slip/invoice inside a media mail package.
You are NOT allowed to have advertisements.
Content Standards for Media Mail
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.
Enclosures in Books Mailed as Media Mail
Enclosures in books mailed at Media Mail prices are subject to these additional standards:
a. Either one envelope or one addressed postcard may be bound into the pages of a book. If also serving as an order form, the envelope or card may be in addition to the order form permitted by 4.2b…
b. One order form may be bound into the pages of a book. If also serving as an envelope or postcard, the order form may be in addition to the envelope or card permitted by 4.2a…
c. Announcements of books may appear as book pages. These announcements must be incidental and exclusively devoted to books, without extraneous advertising of book-related or other materials or services. Announcements may describe the conditions of ordering books and may contain ordering instructions for use with a separate order form. Up to three of these announcements may contain as part of their format a single order form, which may be a postcard. The order forms permitted with these announcements are in addition to order forms that may be enclosed under 4.2a. or 4.2b…
Enclosures and Attachments for both Media Mail and Library Mail
An invoice, whether it also serves as a bill, may be placed either inside a Media Mail or Library Mail piece or in an envelope marked “Invoice Enclosed” and attached to the outside of the piece if the invoice relates solely to the matter with which it is mailed. The invoice may show this information:
a. Names and addresses of the sender and addressee.
b. Names and quantities of the articles enclosed, descriptions of each (e.g., price, tax, style, stock number, size, and quality, and, if defective, nature of defects).
c. Order or file number, date of order, date and manner of shipment, shipping weight, postage paid, and initials or name of packer or checker.
Once again for those who did not get the notice, VIDEO GAMES ARE NO LONGER ALLOWED TO BE SENT VIA MEDIA MAIL.
Please note, that’s only a summary of the DMM for Media Mail.
The clerk is dead wrong.