Your best waiter suggests a topping when there is time. The kiosk suggests one at 12:15 on a Saturday too. Measured on real orders, that one habit is where the money is.
Guests do not pick pricier items. The average paid topping costs about the same at both channels (€3.97 vs €3.56). The difference is how often anyone gets asked.
Peak hour is when the till goes quiet. With a queue building, staff stop suggesting extras. The kiosk shows the topping screen to every guest, including at 12:15 on a Saturday.
Browsing without a queue behind you. Guests take their time, read the pictures, and add the extra egg they would have been too rushed to order out loud.