You’re also investing in the future. Most of us re-order from the same places. Human nature being what it is, I suspect the house that tips gets their food more efficiently than the curmudgeon who shakes their fist at the clouds and bangs on about the delivery fee.
2 to 5 euro depending on size of order. We usually pay it by card when ordering. We ask them to add the tip when ordering and find that we get our food quickly amd piping hot. The delivery guy usually thank us for the prepaid tip.
Works for us!