I usually tip $3-5 for delivery. There is a “bad weather/late at night” bonus though.
I once had my pizza delivered by a man who recited a pizza-delivery filk of the Green Lantern pledge before handing me the box. Having once delivered pizzas myself, I usually tip well, but bumped it up that night because of the added entertainment. He was surprised by the amount and thanked me.
I said, “Hey, I got a pizza AND Green Lantern.” He was even more suprised by that and was delighted that I had recognized the pledge. Said it was only the second time that someone had known. So I got a chance to tip AND make someone’s day. It was cool.
Back when I delivered in the late 90’s anything between $1-$1.99 I considered a good tip. Anything above that was bonus. Customers who usually tipped $2-$4.99 and I would make extra sure your order was right and throw in some extra condiments if you had order buffalo wings or something.
If someone tipped usually $5 or more then there would be a No-Holds-Barred cage match between the drivers to see who gets the delivery no matter how many pizzas were ordered.
These days I tip $3 for a single pie and $5 if I order anything more. Believe me the drivers remember the addresses of the good tippers and most will go out of their way to make sure your order is right.
One time on a weekday late at night I was the only driver left and one of the $5 tippers had placed an order while I was out. I came in the store and saw the address and got all excited. As I was packing up the order I noticed the manager who made the pie had put the wrong type of sausage on it. So I went over and made another pie, threw it in the oven and then called the customer to let them know their pizza was going to be a bout 10 minutes late. We didn’t have the 30 minutes or less thing but when you order late at night your pizza will arrive quickly as there are few other customers. The customer was surprised I called and thanked me.
My manager came out from the back and asked why there was a pizza in th eoven when there was no order and I told him what happened and he thanked me for making another pie and informing the customer. He also said to go ahead and give them the wrong pie just because they are good and frequent customers.
So I show up at the customers house and explained the situation and gave them their stuff and as usual they tipped $5. I was happy because I got a $5 and by going the little extra mile insured that the customer would continue to tip $5 to me and other drivers in the future.
EDIT: Damn that post came out way longer than I intended. Also Yllaria that is the coolest driver ever. I wish I would have thought of something like that in the 2 1/2 years I delivered.
I tip large for the big orders because they’re usually on football parties at my apartment. Parking is pretty tight so he probably had to find a place to park not blocking anyone.
If you’re gonna base tips on logic you run into issues
Like “dead time” going back to the store without a pizza. That is only valid if there was dead time. How can you know, when the guy delivers your pizza, he won’t go back to his car and deliver another one next door. You don’t know, so it’s nearly impossible to qualify.
'Cause logically if you’re tip includes a portion of it for dead time, it must uninclude that for “non-dead time” or else you’re over tipping.
Tipping in bad weather makes no logical sense, as the guy who makes the pizza, the bus driver, the cops, the guy who repairs the cable, don’t make more when the weather is bad.
Should you tip more when it’s a heatwave and less when it’s beautiful?
Should you tip more for two pizzas? Did it cost him any more gas to come with two pizzas, is it any more effort to carry two? (OK maybe if he has to carry like 4 or more pizzas it becomes an issue)
OK I’m being facetious here.
I’m just pointing out the problems with trying to apply logic to tipping.
I do think you should tip the pizza guy, because his salary is based on the fact that he’d get tips, so it’s lower than would be normally.
I just don’t think trying to apply logic to it really works.
I usually tip 2-3 for food delivery. Sometimes if its like 21 total, I’ll just give the guy 25 and call it good because I’m hungry and don’t feel like waiting for the change. 
$3-4 per delivery sounds about right, regardless of quantity. Except if it’s exceptionally much. 1, 2, or 3 pizzas: same tip. If you order $100 of food, probably a larger tip. Same if you live far.
Of course, this is pretty hard to actually lay down a rule for since most pizza places are different. Does the driver have his own car, or a company one? Is he paid hourly as well? Does he get the delivery charge?
I used to work at a pizza joint, I didn’t grind the flour or milk the cows but there were plenty of times I did actually make the pizza I was delivering.
Anytime I got a tip of $3 or more it was a good delivery. I live in a small town however so people don’t tip very well (if at all). Don’t take that last part as meaning my job was easier, lots of my deliveries took me outside of town, occasionally down hard to find gravel roads.
How do you think the uber-rich got rich? 
I normally tip $5 per delivery, whether I order one pizza or 20.
Okay, I never order 20 pizzas, but you get my drift.