A client buys i.e. 2 coca-colas, 2 rocklets and 1 cheetos, the total price is $80 and he has to exceed that amount paying with 5, 10, 20, 50, 100, 200 and maximum 500 ($Ars). But when the total price is excedeed, he has to stop using more bills (never 300, 400, 84, etc). It has to be random, sometimes exceeds by 20, others by 120 and so.