Well, sorry to disappoint you, but keys will be available to buy. We don’t know right now what else will be available to buy or how much keys will costs.
The game’s currency is “keys” (as encountered in previous Chicken Invaders episodes). You can earn keys during the normal course of gameplay, or (optionally) be also able to purchase them. We do not know the cost of keys at this stage; it will be decided after balancing is finished.
Plan Update #01