Organizing giveaways on Facebook - advice needed

I got a task from my boss to make a giveaway of some coupon codes through Facebook. The idea is to limit the offer to people who "like" our FB page (standard fangate) and deal the codes on first-come-first-served basis, with each user being able to get a single unique code. We used OfferPop earlier this year (they have a 30-day trial for one promotion) but that website requires a subscription and my boss is a cheapskate -_-

Is there any free alternative? I've tried Woobox and Rafflecopter, but both offer only random raffles with free plan (raffles being the only option on latter).
Oh and one more thing - is posting a picture with codes as a Fb status still allowed by their new rules? I know there have been some changes but cannot get a confirmation on this.