I find it hard to believe that having to occasionally return to the restaurant to pick up more orders represents such a large percentage of a restaurant's cost.
The cost of the delivery driver is amortized across the total number of deliveries the driver makes. If you can double the number of deliveries, you've halved the cost of delivery.
Having to return to a fixed point every 3rd or 4th delivery seems like it would add up.
You're assuming the delivery driver can be kept busy throughout the entire shift. I doubt there's many places where a driver could do a single pickup and then be kept busy for 6-8 hours straight.
Having to return to a fixed point every 3rd or 4th delivery seems like it would add up.