Promotional giveaways for small retail business?

We have a convenience store with food service. In order to attract new customers, we are considering issuing a coupon for a free sandwich with the purchase of one. We need to know for accounting purposes how we should do this. Do we need to record the sales of the free sandwiches and how does this need to be recorded for accounting purposes? Would we track the number of freebies and write that amount off as marketing? Any info would be greatly appreciated!