I posted a bunch of WL books and several received automatic requests so I immediately printed and wrapped but half an hour after I printed one member contacted me to say she wasn't even sure how the book got posted to her WL because she doesn't want, had never added and she wanted to cancel. It's already marked as mailed as I was just about to leave the house and was going to drop the books off at a post box along the way. The message came in as I was marking them mailed before I turned off my laptop to leave. 

I don't think there is a way to cancel after postage has been printed but maybe someone experienced before? 

nicole here's the info PBS gives regarding this type of situation.

What if I want to cancel a request I made?

• A requestor can cancel a request by clicking  up until the time the sender accesses the address information (= prints the wrapper)
• The request is not submitted to the sender for 1 hr after it is made; this allows for cancellation if you requested the wrong book or change your mind.
  • If the requestor sends a Personal Message to the sender by clicking  on the request, however, that 1-hr delay will be eliminated, and the book request will be submitted immediately at the time the PM is sent.

• The sender can cancel up until the book has been marked mailed; after the book has been marked mailed, the sender will need to contact us to ask us to cancel if this is necessary.

If you request a book and change your mind, but cannot cancel because the sender has already printed the address:

• The sender is not obligated to cancel if you request this
  • if the book has already been mailed in good faith, of course the sender should not cancel.  You will have to mark the book received when you receive it and repost it to your own bookshelf if you don't want it.
  • If the sender cancels, he or she will have to repost the book, and it will go back to the "back of the line" for book requests. 
• If the sender DOES agree to cancel, and there is another copy of the book in the system, your request will be passed to a new sender when the first sender cancels! 
  • If the sender agrees to cancel this request, then:
    • the SENDER can send in a message to us using the feedback link on the contact us page to ask us to cancel, so that your request is not passed along.
    • If the sender cancels WITHOUT contacting us to ask us to do it: you should be sure to cancel the new request for the book that will appear on your account page after the sender cancels.
      • You will be responsible for the new request if your request is passed along--so be sure to do this before the NEW sender prints your address
  • The PBS system will automatically keep trying to get the book for you until YOU tell the system that you don't want it by clicking "Cancel Order".

Thanks Molly. The requestor also sent me the same information so I just contacted PBS and asked if it is even possible to cancel a transaction once the postage has been paid for. 

there is a way to cancel the postage through the post office. If it was media mail it will take several weeks. Also the post office will not refund unless the label has delivery confirmation on it. Call your local post office they can tell you how to do it.

I had virtually the same story from a requestor.  I had already marked it mailed and mailed it. I just told the requestor that they should review their wishlist regularly for errors they may have made or requests they no longer desired or needed.  I have no control over their wish lists.  I no longer mail same day for autorequests for this reason unless requestor sends pm or adds more books to request. Reduces angst on both sides, although i still get annoyed when someone cancels while I'm packaging the next day.