Why was my Amazon package returned as undeliverable?

The customer’s item was perhaps too big to ship to a P.O.Box and had to be shipped to a street address.
The address could be incorrect or outdated and will be returned to Amazon by the unintended recipient.
A carrier such as the U.S. Postal Service does not deliver to some addresses and it would be best to use a P.O.Box for your Amazon order.
Not all items can be shipped to DOS (Department of State) addresses and will be returned as a result.
If the post office box address was entered in a format the system does not recognize, the package may be shipped through a carrier that cannot deliver to the post office.
Orders shipped to a prison may have delivery restrictions associated with the items or shippers.
Carriers will usually make three attempts to deliver a package before the order is returned. If a package contains more than $1300 of merchandise it will require a signature but it is generally up to the driver’s discretion whether a signature is required.
If the package was damaged on its way to the customer, the shipper may return it to Amazon without attempting delivery.
Finally, a recipient that is not expecting a package may refuse it and it will be subsequently returned.