book matches are... "prohibited in international mail, and in domestic mail via air transportation..."

Here's the full regulation:

10.14.3 Matches
Matches are classified as flammable solids. Strike-anywhere matches are prohibited in international and domestic mail. Safety matches (book, card, or strike-on-box) are prohibited in international mail, and in domestic mail via air transportation, but are permitted in domestic mail via surface transportation if:

a. They do not ignite spontaneously under conditions normally incident to transportation or when subjected for 8 consecutive hours to a temperature of 200°F (93°C).

b. They cannot be readily ignited by friction unless struck on their own or a similar box, card, or book.

c. They are tightly packed in a securely sealed primary receptacle to prevent any shifting or movement that could cause accidental ignition by rubbing against adjoining items. The primary receptacle(s) is placed securely within an outer shipping container made of fiberboard, wood, or other equivalent material. Multiple primary receptacles may be placed in a single outer shipping container. The address side of the mailpiece must be marked "Surface Only" or "Surface Mail Only" and "Book Matches," "Strike-on-Card Matches," or "Card Matches," as appropriate. A shipping paper is not required.

d. The gross weight of each mailpiece is not more than 25 pounds.