Just for the record, you can activate an origin game-code on steam also. (Even though it says you can not.)
Just login to your origin account, get the code, use that code in steam as "Add a game". It will confirm with origin, and the game will download and be accessible through steams launcher. (May take 4 hours before it is actually available in steam, due to origins slow activation process. But this way you don't have to download origin, which is just steam with custom buttons.)
How are you selling individual games... You can only register two cards per household. (Or is it email?) Or do you only have two of each available?
:O that is a great piece of advice!! also Its two cards per email