It's being asked more and more: "Add a tip?" The question pops up on checkout screens when we're just grabbing a coffee to go or picking up a lunch order. Starbucks, for instance, added the tip query to its sales system last year. The countertop tip jar full of crumpled singles and loose change seems to have gotten an upgrade. The tip prompt has even been spotted at self-checkout kiosks in places such as airports and stadiums.
