Everybody knows that you tip your waiter 15% of your total bill. For the most part, this system works. If you order a coffee and a danish your bill is small and your tip is small. Order appetizers, drinks, an entree, and desert, your waiter works more and your tip is more. Works pretty well.
On delivery it doesn't work quite as well. A driver's greatest effort is usually driving to your location. Whether you order a small pizza and a coke, or you order 4 x-large pizzas, the driver still does the about the same amount of work. This is why the 15% system does not work for delivery. If you have a small order, and tip a small amount, he makes less than if he was taking another order instead.
A base tip should begin at $3.00. If the order is particularly large, then you tip more. If you have special requests you tip more. If the driver is exceptional you tip more. Most importantly, if you cost the driver time or keep him waiting you tip more. The old adage that time is money definitely holds true for drivers. The more deliveries they take the more money they make.
Nobody can make you tip more except your own moral voice. I'm just letting you know when you give 6.00 on a $5.23 order your service suffers. Drivers talk to each other. They know who the bad tippers are. If you just chose to be a bad tipper then you chose to get poor service. However, it has come to my attention time and time again people think the 15% rule applies to delivery. It simply doesn't.