Buy X (at different times) and Get Y free

Does anyone know a way to set up a free purchase after the purchase of x amount over time? For example: In my store, I developed a card which allows the 13th purchase of a dozen crickets to be free after the purchase of 12 dozen. However, the 12 dozen are purchased at different times. We punch the card for every dozen purchased. When the 12 dozen have been punched, the next dozen is free and the card is kept by the store. I would like to have the system track the cricket dozens purchased by customer and know to provide the 13th free and get rid of the card system.