Customers have up to an hour after the delivery to add a tip, edit it, or delete it. This does allow customers to tip bait, meaning they add a tip to entice a driver to accept their delivery, then remove it afterward. This has been a common practice on Instacart. I'm only aware of one time a customer ever changed the tip amount. Still, other drivers report that tip baiting happens frequently. Uber's tip policy makes it possible for your actual pay to be less than was offered.