Since Amazon have stopped selling this cheap now located another place doing CD key and download of Combined Ops even cheaper.
Combined Ops is ARMA2 original and Operaion Arrowhead expansion. You need both to play DayZ (however you can play with just Arrowhead and the free version of ARMA2 but you get crappy textures).
I signed up for an affiliate account with them today so if anyone feels generous you can access the site via this link: http://www.cdkeyhouse.com/partenaire.php?leemako
or the direct link is here
Buy cd key ARMA II: Combined Operations - 9.99€ - Video Games - Simulation
I applied a discount code I found online at the checkout to drop it a little more (not by much
My order went through instantly (have used CDKeysHere in the past and they manually email the keys which can take a while). Check your JunkMail filter if it hasnt arrived. I had to confirm my order by getting a code txted to my phone which worked no worries.