I've yet to see a QR-code ordering system that let you choose to pay at the end of your meal. The idea of deciding how much to tip before you've even interacted with any of the staff or tasted your meal seems utterly absurd to me - what are you actually tipping? If I were running a restaurant I'd explicitly avoid using a pay-up-front system largely because I'm sure for most customers it would remove any incentive to tip based on the level of service/quality of food provided.