$20 - 20% = $16
ARMA II Operation Arrowhead | PC game | Download discounts at Green Man Gaming
20% off code - PCGMR-GREEN-SUMMR
BTW you won't find a legit Operation Arrowhead key for $10. Only grey market keys will be in the $5-10 range.
BTW GreenManGaming is like steam. So not only will you get a key, you will also be able to download the game as well.