Why wouldn't the fuel be electricity which has no weight? And urban areas use considerably less of it (lower speeds etc). I didn't say the delivery vehicles do exactly one delivery at a time.

The point is that driverless vehicles can be different sizes than current vehicles, and are significantly less constrained because of no human driver. Consequently the delivery strategy can better match financials. They can do things like meet you as you get home. Or wait somewhere for 30 minutes to better match delivery windows. Or charge more for priority. Or deliver at 3am. Or bring vehicles from other areas to match changing demand.

Fewer constraints means more scope for optimisation.