When the customer decides to buy a promotional product, it's best practice when he immediately knows what the bonus is, and even better if he gets it right here and now.
Unfortunately, such promotions are at the same time one of the most expensive for producers, hence they are most often used on a limited scale by selected retailers.
An example of such a promotion from the Circle K chain - the customer wo buys 4 bags of 0.5l of Coca Cola gets a sports bag. In addition, the action is very well thematically related to the World Championships.
At the communication level, the retailer added a completely different mechanism "Buy 2 pcs for PLN 6" and probably slightly overcame it, because it disturbed the clearness of the message. Of course, you can graduate promotion, but it's best to keep the communication and mechanics clear - for example, different gadgets for purchases of different price.
Remember that the average time of eye contact with POS communication is only 1-2 seconds.