That is the big key. The question is not if it crashes, but when it crashes?
Whether it be pilot error, mechanical error, or it being hit by a bird it will come down and any aviator will tell you that altitude is our friend as it allows you time to plan and more room to ditch.
There is a reason why we have a FAA and NTSB and it is to not only control our airspaces but create regs that keep air space and travel safe which is why America has the safety record it has. Here is the other thing you have to look at, if you allow Amazon then you have to allow Best Buy, and some other mom and pop start up business, which sadly can lead to terrorist attacks if in the hands of the wrong person. Regulations have to be put in place that certifies who can fly it, what altitude, what equipment must be on board, and how to protect people and property. There is a reason we have altitude restrictions which is to protect people and property. Lastly people say this will be tried in a large metropolitan area, yeah that is a great idea try it out where you have more chances of harming people or property. It would actually be wise to try it in a rural area where if it has a mechanical failure it can deck in the woods somewhere.
Sorry, but I just do not think our greed is worth public safety. Especially when you can get in your car and drive to a store.
One area though that you can try this out is at bars or restaurants where if it fails it only harms the patrons and property inside the building.