Instead of a battery-powered beacon you would likely use fiducials [0].
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.
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.