You can avoid this and all the quasi-legal unregulated airspace drama by... wait for it... just being regulated and flying higher up in regulated airspace! Sure, meeting FAA regulation is hard, but it's necessary and I'd be very confident that's the right route for a service like this. If they fly high enough the only point it can be shot down is over Amazon's depots when they pick up goods, and over the house of the recipient when it drops them off. The former is well isolated, and the latter is unpredictable from the point of view of the hypothetical shooter.