Spit is not one of my favorite pizza toppings, so usually around 5 bucks.
Don't you tip when he hands it to you, or after? How could that individual spit in it then? Just saying. I've never personally tipped anyone since I never got the door and didn't pay for the pizza.
Once those delivery guys know that a certain address never tips, they do terrible things to that food on the drive over. I guess if you never order form the same pizza place twice, not tipping would be perfectly safe.