Hello!  I recently mailed a novel using PBS's pre-printed wrappers with delivery confirmation.  Today I received the parcel back with this sticker on the front:

Undeliverable As Addressed Unable To Forward

I used PBS to print the label and delivery confirmation, so there can be no mistakes that I have made with the address.  Perhaps the requestor entered it incorrectly?  I PM'd the person and informed them of the situation.

I already paid for postage once.  I am not obliged to resend this book at my expense, am I?

No. The requestor is responsible for making sure the address used is current and correct. The post office does not forward media mail. If they moved and didn't change their address here, they're out a book.

From the help docs:

If you are moving to a new mailing address:  You should not request books during the two-week period before a move, to ensure that the books don't arrive at your prior address. 

• You should also make sure that you have NO books set to auto-request on your Wish List.

If books requested by you are  already "en route" to you when you move:

• You must make arrangements to collect them from the previous address.
• You can also contact your previous PO to arrange for forwarding.
  • This may mean that the package will arrive postage-due, since Media mail does not routinely include forwarding service - you will need to pay the postage due.
• If you have requested books that have not been mailed yet, you can try to "catch" the sender using the PM button on the transaction to provide your new address. 

It is the requestor's responsibility to provide a working address to senders.  If you do not keep your address current and correct on your account, you will need to give credits for (=mark received) any books that are sent to the wrong address, whether or not you are able to collect them, and whether or not they are returned to the sender.   Providing a dependable, correct address to senders is part of the honor system that helps keep this club working so well, and it is essential to maintaining membership.

You may want to pass that on to them. They actually still have to mark the book received.

Do feel free to report the address problem to PBS.  They can see if it was an auto-request.  If it was, then they can put the account on hold until they hear back from the member.

Ask the requestor about the address problem.  If they say that the address is fine -- that there's been no change in the address -- then it was probably a USPS mistake.  If that's the case, the Post Office will send it again for you "under separate cover."  You won't have to pay.

I've had this happen twice.

In the first instance I PM'd the person and they forwarded me another credit to cover mailing it a second time to a corrected address (and then marked it as received when it got there, still well before the system dropped the request as lost).

In the second instance, it was a Wish-Listed book.  I sent a PM to the member asking for them to respond back within a week (I'd learned a little more by this time).  There was no response.  As I didn't want to repost it to that same member (who was again at the front of the Wish List as the system had "lost" the book), I contacted TPTB at PBS and they took care of the whole issue so that I could post the book and the next person in line could receive it.

Hope my experiences help you.

Media mail IS forwardable.  It is forwarded postage due.  The endorsement Undeliverable As Addressed Unable To Forward can indicate several things.  Most commonly, the person receiving the book used to live at that address, but moved from there more than 1 year ago.  Other than that, it can indicate that there is some unspecified error in the address.