I've got a suspicion that Amazon has been trialling/building this in the UK since their purchase of Yodel, perhaps earlier. A number of times some bloke in a car has turned up and delivered me a package. No uniform, no branding. Just a phone running Amazon software. I can't say it's the greatest experience.
But as to it not being the greatest experience, well, yes they are not always as professional as real courier companies. And a lot of the time they've just said "User24?" while handing me the package, so Amazon probably need to put a bit more effort into training. And I wonder how easy it would be to provide fake ID to Amazon and just steal a whole load of admittedly random packages? But, maybe that was the point of this possible trial, to identify issues like that so that the launch experience would be better?