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The only way that trasnactions can be canceled after the postage is printed is when the system automatically does it if the books are not marked mailed. The system will automatically cancel a transaction if it is not marked mailed within the timeframe specified on the request.
When the system automatically cancels an unmailed transaction, the book request is passed onto a new sender. So, if the book is available at PBS, your requestor has a second order for the book already pending for them.
Your best bet at this point is to contact the requestor to let them know you are ready to mail the book out, and make sure that they say it's OK to mail it to them first, (that way they can cancel the 2nd book order that they probably have). Then mail the book and when the requestor receives it, they can mark it received from their transaction archive, thus giving you the credit.
If they can't cancel their second order for the book, or if they don't want it for some reason, then you are just out the postage. Sorry.
A requestor cannot cancel a book transaction once the address has been accessed by the sender.
If they PM'd you asking to cancel after you printed postage then you do not have to cancel and lose your postage.
If you forgot to mark it mailed then you are out of unluck unless like Sara says, the requestor agrees to and is able to cancel any other pending requests for the book and wait for you to send yours.
This is why it's a good idea to mark books mailed when you print the postage. On the rare occassion that I use printed postage, I always have the book in hand to douple check it before I buy the postage and I mark the book mailed right then.