The bad thing about choosing "Return to Sender" is that you have to pay for return postage. If you aren't sure, why not put an address of another contact in the same country.
A few years ago before I knew about AnySoldier.com, I had sent some boxes to the son of a friend of mine. I researched and found a site that said to mark the boxes something like this. "If soldier has redeployed, DO NOT RETURN TO SENDER, send to commander and redistribute to troops."
I think it would be good to put a big "DO NOT RETURN TO SENDER", redirect as instructed on customs form.
Sometimes they have forwarded packages to troops who have returned home and they receive them months later.
Very proud daughter of a WWII Fighter Pilot, Niece of a Vietnam War Fighter Pilot, Cousin to a Naval Fighter Pilot and Navigator! Go U.S.A.