This is a great feature and I would also try to incorporate a smart calculator that tells you how much you have to pay for service fee (as it is included last after your overall bill) so you wouldn’t have to further calculate yourself. You could even add one for VAT if you want.
When there’s no service fee, things are easier as people don’t need to make additional calculations on what they have to pay for when they have ordered more one item (perhaps an integrated calculator where it adds, muitplies etc within the app could be useful so you wouldn’t have to get out of the app)
Subtotal Total: £7
Service fee 10%
The guy paying for coke could just be prompted to pay £2.10 straightaway instead of him doing the manual calculation.