Many parts of Europe solved it in a more low-tech way: street-side parcel lockers unlocked with your phone. Massively reduces delivery cost (i.e., one driver can deliver far more packages per hour), is pretty convenient and safe (no packages left unattended), and best of all, doesn't require a fleet of UAVs.
You can pretty naturally extend this once you have self-driving vans.
I think Amazon has a proprietary version of this in some parts of the US, but at least where I live, the lockers are a car drive away, which defeats the purpose.