I have two remarks or questions. The person who receives the package doesn't have to (or cannot) sign for it apparently. How difficult is it to shoot down such a drone to get the loot, if you will. Might become a sport for some folks.
Considering how signatures are mostly a farce (I routinely sign for my wife or my neighbours, who do the same for me), that's not a big deal: you'll just choose "classic" delivery via courier if you're really worried about that. Interception is a real problem though, especially in high-crime areas. You could reduce it with some sort of hardened container which can be opened only with a certain code.
If they drop it next door, they note which house they dropped it at and also have a signature to show someone actually took it.
The point is not to show that a particular person received the package, but that someone received the package. 99.9% of the time that's enough. If it wasn't and was impacting their bottom line, they'd have started taking photos of recipients by now.