http://en.wikipedia.org/wiki/Military_mail

The United States Postal System has special discounted rates to send mail (including tracts) to soldiers with a military postal address.
Please visit your local United States Postal System office to find out about the low rates.

Military Mail on Wikipedia
http://en.wikipedia.org/wiki/Military_mail

United States Postal Service and Military Mail
http://www.usps.com/supportingourtroops/welcome.htm
http://www.usps.com/supportingourtroops/addressingtips.htm
http://www.usps.com/supportingourtroops/mailingrestrictions.htm