I don't understand why tipping a delivery person should be based on the total of your purchase. It takes the same amount of time, creates the same wear & tear on the vehicle, and uses the same amount of gas to deliver $2 worth of food or $100. I base a delivery tip on the distance from the restaurant I order from and the weather conditions outside. If the roads are dicey, I always tip extra.
I agree with you. Tips based on a % of the bill seem crazy.