I am explaining to a requester ( evidently a recent member) that they cannot cancel after the sender has printed the postage, and why this is a good rule.However I cannot find it in the PBS rules. 

could someone help me find it please?

can you get a refund for an unused label?

http://www.paperbackswap.com/help/search.php?terms=cancel+after+postage

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".

What if I print with postage and then can't send the book, or my printer malfunctions?

• If you cancel a transaction (or the system cancels the transaction if you did not mark the book mailed in time) after printing with postage, the postage and fees can't be refunded by PBS.  The funds are transmitted to USPS when you download the wrapper with the postage on it.  You could try taking the printed, unused wrapper to the PO and discussing this with them.    We have no mechanism to recover these funds from USPS ourselves, but individuals may be able to do so.

I have heard that a label must have delivery confirm on it to get a refund. The dc is how the post office can determine whether the label has been used or not.

Since you have already paid for postage, about the only other fair option would be to have the requester buddy you a credit as reimbursement.

Charles, IIRC all PBS postage labels are printed with DC/Tracking.  Refunds are trixy and not guaranteed, sometimes PBS admin can do so if the request is made...within 24 hours?

ETA - It's heavily discouraged for the requestor to request a cancellation after the label has been printed (especially with postage) ... which is why the 'cancel transaction' button is removed from their home page at that point.

A Buyers Remorse/waiting period is built into the PBS FIFO system, and it's not intended to take place after shipping labels are printed.

If you get your postage from  paypal.com/shipnow, you can get the postage refunded if you don't use it. It may take a few days, but you can get a refund.

The requestor is out of luck as the postage was already paid for. I know I wouldn't cancel a transaction I already purchased postage to mail. I'd just tell the requestor to repost the book if they no longer want it.

most of you are dancing around the question.  It is NOT POSSIBLE for the requestor to cancel after the label is posted, that is one of hte safeties in place by the site.  Just like the "buyer's remorse" request delay.

The reason it's a good idea, however, is so the sender does not buy postage & send out a book in good faith, then lose their credit if the request is THEN cancelled.