One problem with Zippy would be it contending with a myriad of laws stemming from whether Zippy is a vehicle, especially a motorized vehicle on a sidewalk. Let's ignore the unmanned part for now.

"Having small items delivered to homes by robots gives us near zero cost per delivery, "

You would still have variable costs like fuel, which would be incurred on a delivery by delivery basis. Then how high is Zippy's fixed costs?

I suspect that Zippy could only work in certain very densely populated areas, where users are willing to make concessions. In a suburb, Zippy because it using a sidewalk, would be too slow, but could do front door delivery. For an apartment building, Zippy could only go to the lobby's front door, hence requiring a concession from the customer that they may not want to make. Also how far can Zippy operate from the restaurant in question (it has to travel back)? What is its payload?

When I come to think about it, Zippy would be analogous to a person in a wheelchair making deliveries.

I suspect one could use computer simulations and some what-ifs on a spreadsheet to figure out Zippy's feasibility. I would wager that a person in car is simply more flexible, cost-effective and provides a more satisfying experience to the customer.