Today a book I mailed to Michigan was returned back to me by the post office.  The post office put a label on the package that says "RETURN TO SENDER; UNABLE TO FORWARD" and I don't know if this means anything but they wrote U+F and circled it in red ink.  The book label is the Media Mail address label I got from the PBS system.
What am I supposed to do now?
I paid $2.47 in postage to send this book and I have it back now.  There's a USPS tracking number on it too.
 

Ok, this gets even weirder.  I just checked my Tranasction Archive and the person has marked the book received! 

It's possible also that the site marked the book received if they've become aware of an address problem.  This help doc has a lot of good info:

A book I sent was returned to me!

If the book has NOT been declared lost in the mail yet: you should contact the requestor using the button on the active request on your My Account main page, to let him/her know that the book was returned, and to confirm the address. 

• You should not re-send the book if it is damaged.  If it is damaged, contact us to cancel the transaction.

  • If the book has already been declared Lost in the Mail, you do not need to contact us to cancel.

• You are not obligated to re-send the book to the requestor if this happened due to a problem with the requestor's address.   If the book was not damaged when it came back to you, and you do not want to re-send the book to the previous requestor, you can simply repost it. 

• • If you do not get a reply message from the requestor within 5 days of your PM, contact us.

If you mailed with Printable Postage and you need to generate a new wrapper:

• Since you can't print postage again - it's illegal to use printed postage twice - you need to use the "mail to a friend" option to create a new postage-paid wrapper.
  • If the return was USPS's fault, you can mail "under cover" to the other PO for no additional postage, so you should take the package to your local PO and explain what happened.
• To generate a new postage-paid wrapper by creating a "mail-to-a-friend" (noncredit) swap for the book:
  • Detailed Instructions on how to do this are in Can I send a book to a friend/alternate address?
  • Basically, you choose this option from the More Options menu on the Book Details page and supply the name/address, then choose postage when printing.
  • This will print new postage and you can use that to remail the book.

If the book HAS been declared Lost in the Mail: you should contact the requestor to find out what happened to cause the book to be returned (if the reason is not clear) and to make sure that the requestor still wants the book (and that the address is correct) before remailing it. You can use the button on the "lost" transaction in your Transaction Archive (linked from the top of your My Account page) to send a message to the requestor.

• If the requestor no longer wants the book (if he or she has gotten another copy from someone else after yours was declared lost, for example), you should REPOST it to offer it to the rest of the club for swapping.

•  If you do not get a reply to your PM

  • do not re-mail your book - you should repost it to your Bookshelf if you do not get consent from the requestor to re-mail the book.  If you repost and the new request is from the same member, contact us for help.

• If you do not receive a reply to your PM within 5 days, contact us for help.

The most common causes for packages to be returned to sender:

• The package was over 13 ounces and bore only stamps for postage, and you did not mail it by handing it to a USPS employee.
  • This is a USPS regulation if postage stamps are used for postage and the package is over 13 oz in weight.
  • The regulation does not apply to packages sent with printed postage
  • You can remail the package from the PO in this case, without adding new postage to the package.

• Incorrect Address. Caused by 
  • transcription errors if you hand-wrote the address
  • the requestor mistyping his/her address in his/her account
  • USPS error

• Insufficient Postage Caused by 
  • Insufficient postage on the package
    • did you check and adjust the weight on the wrapper settings page before printing? this will ensure that the correct postage and postal rate will print out
  • Failure to write MediaMail on the package if the PBS Wrapper was not used.
    • USPS will charge First Class rate if MediaMail is not specified. 
    • The PBS Wrapper will print out with FIRST CLASS for packages weighing 7ounces or less; packages over that weight will print wrappers marked MEDIA MAIL
  • Writing First Class on the package if the PBS Wrapper was not used.
  • Reusing Priority Mail materials (this is not permitted by USPS unless you are paying priority rate!) but applying Media Mail or First Class postage.
  • USPS error.
• Moved, no forwarding address Caused by
  • The requestor moving before the book arrived (Media Mail is not routinely forwarded, unlike First Class mail!)
  • USPS error.
• Address illegible Caused by
  • the wrapper tearing and the TO address being lost or obliterated
    • to prevent this problem, consult the Help Center document All Help documents about bookwrapping to improve Wrapping technique
  • printer ink running because of exposure to the elements
    • to prevent this problem, you can apply a layer of clear shipping tape over the printed address portion of the wrapper.  Remember not to tape over stamps!
  • Poor handwriting (if you hand-wrote the wrapper)
    • to prevent this problem, carefully copy the address information clearly onto the wrapper - or print the wrapper from a printer.

If the book was returned to you through an error on the requestor's part, you can ask him or her for an extra credit (or money for the postage) to re-send the book,

If the book was returned to you through USPS error, you can take it to the PO in the packaging in which it was returned, and explain the situation - that it was returned in error from a current, deliverable address.  They can check the address and if it is correct and the book was returned through USPS error, they can send it again "under cover" from Post Office to Post Office, without requiring more postage.

If the requestor still wants the book, and you re-send it, it may be declared Lost in the Mail before it arrives. 

• The requestor can mark it received from the Lost transaction in his or her Transaction Archive, and you will get credit.
• When she or he does this, if he or she notes the postmark, you may get a "late postmark" email - the system can't tell that the late postmark is from RE mailing the book.  Don't worry if you get one of these emails - as long as you do not have a habit of late mailing, one of these emails will not affect your account at all.

Thank you!  I'll read that. 

UTF = Unable To Forward is the endorsement we use now instead of Forward Order Expired, and also for a few other situations.