A long time ago when I worked for a middle man delivery service (Before grub hub, etc were big and entrenched) I loved people who ordered a sandwich and tipped $5. The percentage of the price to tip was extreme and that is typical the metric used. Sure I’d like more money if offered but I think this is fine. Tipping is weird but this is how it works.
What I hated was huge orders where they didn’t tip. I had to double check everything because the restaurant staff were sometimes openly hostile to us, and the bigger the order, the longer it takes.
The biggest order I ever delivered was about $350 of food. The place I delivered too was like a cartoon. A huge mansion set back a mile long driveway and I was literally surrounded by hounds as I came down the driveway. They had a fountain and a huge cul de sac and a half dozen luxury cars.
They didn’t tip a dime.
The best tip I ever got was from a guy who was high out of his mind and just handed me a bag full of change. It was $56.