I don't do much mailing of matchbooks or matchcovers myself, since I'm building my collection and not mailing to other collectors, but I do know that lots of eBay sellers are still mailing full books of matches. I normally strip the matches out of the covers when I get them unless they're older books. So, if I happen to mail some matchcovers to a new collector or slip them in with chips as a freebie, there usually aren't any matches included.

There IS a procedure for mailing safety matches, such as matchbooks, by ground transporation domestically within the U.S. The following link is to a USPS publication which describes how to do so (under Packaging Instruction 4B):

http://pe.usps.com/cpim/ftp/pubs/Pub52/pub52c.pdf