Well, if you did this, you are not the first, and you probably won't be the last!  The first thing you should do is let the requestor know what happened, by sending a Personal Message: click on the active transaction for the book on your My Account page to do this.  Members are usually very understanding about such things--everyone makes mistakes! But if a package arrives postage-due without warning it can be irritating.  Best to let the requestor know that this may happen.

All you can do after that is wait and see what happens.

Several things could happen from here:

• The USPS will return your package to you.
  • This might actually cost you postage, as Media mail rate does NOT include the cost of returning the package to the sender.
  • But if the package is returned to you in the first few days, you are not usually charged postage (because it really didn't go anywhere).
    • If this happens, you can just put the postage on it and send it out!
  • If the book is returned to you, and it has been already been declared lost in the Mail at PBS:
    • You should ask the requestor if she would like you to resend the book.
    • If she has already gotten a copy from another member, she will not want a second one
    • if she has not gotten a copy, you can resend the book,
    • When she gets it, she can mark it received from the Lost Transaction in her Transaction Archive, and you will get credit for sending it when she does.
    • If she does not want the book, you should repost it.
• The USPS may deliver the package to the requestor postage-due.
  • She may pay it, and contact you via PM to ask for reimbursement.
    • There is not a way to transfer PBS Money between accounts but
    • You can send reimbursement via snail mail or PayPal - whatever the two of you decide
  • Or she may refuse the package, and it will come back to you (postage-due again; you will probably have to pay postage to reclaim it)
• Occasionally the package will not be delivered OR returned.
  • Some Media Mail packages sent without postage will be destroyed.
  • We think this happens very rarely.
  • If this happens to your book, it will be unreceived by the system action date, and will be declared Lost in the Mail.  You will not earn credit for an unreceived book unless you used PBS Delivery Confirmation and the barcode was scanned by USPS.
• Sometimes (not often), the USPS does not even notice, and delivers the package (NOT postage-due).

Related Links:

How much does it cost to mail a book?
Do I have to mail from a post office?
How do I contact another member?
Can I see my past transactions?
Does the postage print out on the wrapper?

What is PBS Money?