I'm very doubtful of drone delivery until battery tech advances at least 10x.

In dense, built-up areas there is no space to land. You can't fly between buildings and land on a sidewalk in Manhattan, and where would you leave the package there?

In rural and suburban areas, where houses have backyards, the flight range starts to be a problem. Amazon isn't going to build a full warehouse every 5-10km, even "drone depots" seem doubtful with that density. With "drone depots" instead of proper warehouses, the packages would still have to be trucked in, so 1-2 day delivery instead of 1-2 hour delivery, and the only motivation would be cost saving over traditional mail carriers.

Perhaps if batteries become a lot more capable, then having drone depots every 20-50km in suburbia might end up profitable over paying for mail.