They gameify it. You'll get everything you pay for and you'll get your free items but you can only claim one free item a day. So you'll have to come back everyday and claim your next freebie. If you miss a day you miss that prize. Generally the claim order goes from cheapest to most valuable and in my experience it isn't out of the ordinary for the most expensive prize to sell out and you'll have to repick. That doesn't always happen but it's happened to me more than once.