Never had this happen before and it still may work out.  I accepted a request for a book I KNOW I have but it isn't where it should be.  I've already printed the label.  Should I have to cancel this request, will I get my postage $$ back or is it just a loss?  Thanks so much!

Contact PBS as soon as possible; you may be able to get a refund but it may depend on timing. You usually can’t cancel after printing postage without getting PBS to intervene since PBS pays the vendor.

What Linda said. The Post Office gives the site a small window during which they can refund postage, so contact them right away. Also, the Post Office doesn't have to accept postage that's over 48 hours old (although they often let it slide) so better to have the book in-hand before printing.