I like the placards placed in "safe landing zones". Simple solution to hard problem.
*found the thread https://news.ycombinator.com/item?id=6833223
"This is just one idea. Perhaps Amazon provides you with a beacon for the drone, or you get a big QR code for the drone to target your house & they don't need pilots at all. Who knows!" -owenmarshall
1) If you can see over the fence...
2) If it shows up in Google Maps ;)
#2 might be implausible now, but if people get really accustomed to this, I guarantee that some handymen are going to tile/spraypaint large, permanent drop points in their yards.