The drones could hitch a lift on people's autonomous vehicles. Amazon would give you a roof rack and pay you when it rides on; they can hook in to your GPS and schedule to anticipate routing.
We all have windows - and many of those windows open. A basket that hangs outside the window with a battery powered beacon is another option for delivery. The baskets could even be secure and waterproof.
The issue I see with delivery to windows is all the clutter that is commonly found in front of windows such as power lines, tree branches etc. since that cannot yet be easily detected with computer vision and inexpensive 3D sensors.