My guess is they could request the item back, and if they really wanted to, put a negative balance on your account effectively banning you from Amazon without a return. Whether or not they would sue you or win and collect is a different story.
All of this is assuming this was not an intentional PR test by Amazon for Prime day. One would presume that that is possible.
As I understand this if it lands on your doorstep and wasn't reclaimed by UPS/FedEx/USPS prior to that there's nothing they can do. They can ask you to send it back, but you don't have to. If they made the transaction and it's in your hands they can't put you in the red for the difference.
Correct. Now, I highly doubt they'd do it, but Amazon could take some action such as banning you from their services until you return the item. At which point do you want to own the item more than using Amazon? Could be a tough choice for some.
All of this is assuming this was not an intentional PR test by Amazon for Prime day. One would presume that that is possible.