From what I have gathered a postal match is one where you shoot localy at your own range. Your targets are then verified (with Sig) by an NRA member, a range officer or a KTOG member with the appropriate number of posts. Targets are then sent in via mail and judged as if everyone had shot together on the same day at the same location.
you can read the info from 2007 here
http://www.ktog.org/cgi-bin/yabb2/YaBB.pl?num=1176592833 I don't think there is much intrest in this anymore.