Media Mail is for books only.
Media Mail is the cheapest way to ship books, and it's how most Second Date parcels travel. The eligibility rule is narrow, and Media Mail is not sealed against postal inspection — USPS may open the package, and ineligible contents can arrive postage-due or be returned.
What can go in the package
The book (or books) in the sale — books of at least 8 printed pages, reading matter, with no advertising other than incidental announcements of other books. Alongside the book, only paper:
- A packing slip or invoice relating to the contents (DMM 173 §6.3).
- A short personal note or greeting card — an incidental personal message included with the parcel (DMM 173 §6.4). Postage is on the combined weight.
- A plain paper bookmark — loose printed paper of the kind mailable as USPS Marketing Mail (DMM 173 §6.1).
What cannot go in the package
- Merchandise or gifts of any kind — candy, tea, stickers, pens, shop merch, freebies. Anything that isn't paper is not permitted.
- Advertising inserts.
- Personal correspondence beyond the incidental greeting — a real letter needs First-Class postage on top.
The practical rule
The book plus paper only. Notes and bookmarks are fine; anything that isn't paper is not. If you're unsure whether something qualifies, leave it out — the cheap rate for the whole parcel rides on it.
This list follows USPS Domestic Mail Manual 173, §§3–6. The buyer-facing version of the same rule is on the how it works page.