$20 for 2 meals, then $4 would be a reasonable tip. If I go to a fancy expensive restaurant and spend $200 for 2 meals, then the tip would be $40. The server at both places are providing the same service with likely similar effort/difficulty, but is getting 10x as much just because the food costs more at one place.

With delivery it's even worse I think. If I order 1 pizza, then I tip the delivery person based on the price of that 1 pizza. If I order 3 pizzas from that same place, then the tip is based on the price of 3 pizzas, but there is very little difference (if any) in the amount of work for the delivery driver between delivering 1 pizza vs 3 pizzas to the same location.