Umm, if people actually tip the drivers, are they really doing it via some app or Amazon website with a credit card? Really? How else does Amazon get to even know there was a tip, let alone how much and that they can use it? Surely anyone tipping a driver (and yeah, it was a real revelation to me, too) would use cash, and it would go straight into the driver’s pocket.
Maybe that’s what the tip is for - that they actually rang the bell and hung around for the door to be answered. Rarely happens round here.