It is also clear to me how this is implemented: Available items are all in local Amazon warehouse. The delivery is done by usual Joe the person who has car and can go around one neighborhood. About $8 is recommended tip for the delivery for order < $100. Assuming Joe the person would be going 20 or so deliveries in a small neighborhood, this would take him 5-6 hours. The expected tips alone would make up for minimum wages and fuel expenses.

In all delivery problems, last mile is the most expensive miles and solving it distributedly using local drivers seems like idea even more brilliant than calling cab via phone. You don't need to drag accountants.