Imagine you're in line at a coffee shop. You order your usual cappuccino and swipe your credit card to pay. Then the cashier swivels a little screen that prompts you for a tip – before the espresso shot is pulled or a drop of milk steamed.

Do you tip more, perhaps hoping that it will lead to a better drink? Or less or none at all, peeved at being asked to reward service that hasn't happened yet? Do you feel pressured into tipping the suggested amounts, which can equate to more than half the price of the drink?