More than direct-to-customer deliveries, I see Amazon using this for a next-to-the-last-mile distribution network. Basically, extending their distribution network out one level to the retail storefront scale. In this model, drones would take packages from regional distribution centers to customer-facing pickup locations (i.e. the retail storefront). In such a network, the drone could drop the package in a chute on the roof of the pickup facility, to be manually or automatically sorted for pickup. In a fully automated system, you could use robotics to move the package from the receiving area to a passcode-protected pickup box (like a post office box).
The customer experience would be:
1. buy on Amazon.com
2. receive order confirmation with access code
3. wait some time (4 hours?)
4. travel to pickup location
5. locate box & enter access code
6. retrieve package
Technically much simpler than figuring out doorstep delivery (which has endless complexity... single family detached homes, multi-unit dwellings, awnings/hanging pots/other obstacles, etc.) and it keeps the drones out of "people space".