Around $3, for an average size order. A little more if we have a very large order that the delivery person would need to juggle.
That seems adequate to me for the work being done on my behalf. I don't think 20% is a resonable standard for delivery. It's fine for restaurant service, but food delivery is a completely different animal.
I do take the delivery charge into account when I decide on a tip. What percentage of the delivery charge goes to the delivery person is between them and the store, but if I'm expected to pay a delivery charge, I expect it to offset some of my delivery costs.