I delivered pizza and I didn't really give a fukk if people didn't tip and no one else I worked with did either. If you come to the door right away and have the money ready, no delivery guy is going to trip about whether or not you tipped. A lot of people don't really know it's customary and probably close to half of the people never tipped. You make so much money from the people that do tip that you don't care. The ones that got flagged were the ones that would habitually order the pizza and then leave and we'd have to wait around for them. When I got one of those deliveries, no matter how close the place was, I'd always do all of my other deliveries first since they probably wouldn't be ready anyway.
Some people tip big because they think it'll make their pizza come faster next time but I think it really works in the opposite way. At the place I worked, you had to take whatever was next in line but you could add others on top of that. If you were known as a good tipper, some driver might be a jew and take yours even though he was going to the other side of town and your pizza will be sitting in his car for a half hour. They know you're tipping out of habit and not for good service.