You will receive your credit back when the system declares the book Lost in the Mail. If the other member used PBS Printable Postage or PBS DC they have already received their credit. If they used another type of Delivery Confirmation, then they will not get their credit until the book is marked Received. Some PO's scan the book's bar code as delivered before they deliver the book, some Mail Carriers have hand held scanners they use as they deliver the book. The book may have been delivered to a wrong address. There is a link in the Help Center to the USPS website for lost or stolen packages, you may want to have the sender fill out the form to see it the PO can trace the package. The link is in the Help Center Doc: A Book I Sent Is Lost In the Mail.
It may show up though, and if it does come after it is gone from your Main Account page, you can mark it received from your Transaction Archive.