Ok, now this is where i sound totally callous and mean, but....
It's not my responsibility to make sure that a delivery drivers wage is enough to live on, in fact its probably down to people who do tip why their employers are getting away with giving them such an abysmal basic wage.
I've initially paid for the pizza, and its usually way more than what the damn thing is worth to begin with, i accept this and pay the cost anyway, i will not pay additional for that item to be brought to me, given that the cost of the delivery is worked into the cost in the first place.