The practical answer is probably to blend the technologies. In an urban environment, the bikes do the last mile. Very small drop off points with tiny specialized helipads would accept a stream of fast deliveries from the more distant real warehouses. Since the routes would be established, the computer technology demands would be modest.

Get the package to the drop off point in 30-60 minutes by quad-copter. Get the package from there by bike in 30-60 minutes. There would be uncertainty based on the availability of human messengers, but routes and delivery times can be estimated/calculated by computer. "Estimated delivery in 79 minutes. Click yes?"