When someone says, "Package delivery via drone," we have an image of a drone quad-copter landing in our backyard or some other such place and depositing a package. That is one scenario, but Amazon is working on a bunch of different solutions, one of which is described in this Puget Sound Business Journal article, Amazon patents fly-by drone package delivery technology.
Think of packages dropping to earth via a multitude of ways. Certainly having something that can deliver multiple packages and do so accurately is what everyone will want. It is going to be interesting to see what they come up with. I'm betting they will try multiple systems to see which one works best before rolling out a drone delivery system worldwide.
Expect to see that system launched in a country other than the United States — we are far behind on the commercialization of drones.