> The marginal cost of one delivery is obviously not 2 man-hours, that'd be ridiculous.
Why not? 1 hour to walk around the store and collect everything, 20 minutes to check it out, load the car, drive the the person's house, 10 minutes to walk the groceries to their apartment, wait for them to sign and open the door, 20 minutes to drive back. At least 2 hours of work.
Of course if you run a delivery service in the most idiotic way, it'll cost you a few man-hours per delivery. But we're talking about supermarket chains, i.e. people who know a thing or two about preparing orders and dispatching them efficiently.
Why not? 1 hour to walk around the store and collect everything, 20 minutes to check it out, load the car, drive the the person's house, 10 minutes to walk the groceries to their apartment, wait for them to sign and open the door, 20 minutes to drive back. At least 2 hours of work.