Haven't delt with this program, but MN has a couple of programs sort of like it, was really nice during this last deployment to receive packages...I was horrible about returning correspondence which I still feel guilty over.
Just a note, with all the digital technology set up often its easier for a soldier over there to email than it is to respond to snail mail. Even if you send snail mail adding an email address would greatly improve your chances of being contacted and thanked properly. Just know that through all of the soldiers i've had over the last 8yrs even if they didn't respond know you DEFIANTLY appreciated.