I don't know if people have already said this, I've just noticed some things in these comments. I used to work at a pizza franchise and did any job you could think of while there.
From my experience, driver's don't usually get the delivery fee, that is merely the store's driver safety insurance charge. The store gets that money, so they have a way of reimbursing driver's if they ever get mugged or something. Delivering can be dangerous if you're near a shady area.
Other delivery services/stores may vary, but my driver's made below minimum wage. 2 maybe even 3 dollars is the least you should give no matter what the cost of your order. Gas is costly, and you should pay them for delivering food to your doorstep.
If you order delivery due to inclement weather, give them extra ( 150% - 200% what you would normally give, i.e. normally $5, give $8 or something )
Some courteous things to do when you order delivery:
Turn on porch light if it's nighttime
Don't make them wait in shitty weather ( be on the look out for them )
Have your money ready
Don't pay in coins