I’ll put it this way… for dine in tipping, 20% is fine. If you order a cheap meal by yourself at a restaurant, that $4 tip on a $20 meal is fine. The server probably didn’t have to spend more than a few minutes with you.
If you are a table of 5 with a bunch of drinks and a $200 tab, the server probably earned their 20% of $40.
For delivery, a flat rate makes more sense. If someone delivers 3 pizzas and some wings for $100, did that take much more effort than delivering 1 pizza for $20? Same number of steps taken, miles driven, gas used, time used, etc.
$8 to $10 makes sense for doorstep delivery in todays economy. $5 was fair pre-pandemic.
If you are getting a whole bunch of stuff delivered then I can see justifying a bigger tip, but probably not percentage based.
A $4 tip on delivery means the driver is taking a loss or maybe breaking even. They shouldn’t have to suffer because you had a small order.
The service you receive for delivery is not as directly correlated with the total ticket amount as much as dine in might be.