I just got off the phone with the USPS hotline and forgot to ask her where I can find this info online.

Troops stationed overseas are able to send First Class Mail for free (her understanding was this meant letters & postcards, not parcel).

I'd like to print something out to include in my care package since I was planning to include a roll of postcard stamps to send along with postcards for them to mail to their families & friends. 

RAH, RAH RAH!!  GO USPS!!!!

Google is your friend.  "troops mail free" gets many results, including this one:

http://fas.org/sgp/crs/misc/RS22203.pdf

This is an updated version of the info I found.  Thanks.  I thought someone who works at the PO might know if it's still in effect.  (And if it had an easier to read quickly.)

If my Google & your Google are getting the same results, I'm going with it.  (And my friend was chatting with me last night -- he got an R&R day with unlimited Internet use -- and he said he thought that everyone is probably eligible but not everyone knows about it.)

"Members of the armed forces on duty in designated combat areas can send personal

correspondence, free of postage, to addresses in the United States."

"on duty in designated combat areas" -- not all troops qualify

"personal correspondence, free of postage" -- that means letters only