How about combine this with book-in-advance? So say you get $20 of pizza for $10, coupon website actually runs the payment system - takes payment of $10 in advance, and then you show up to get your deal. Then limited-capacity places could do realtime deals available to limited number of people to reach but not exceed their capacity, and bargain-hunters would be sure they'd get their deal and not show up somewhere that just became full.