I just received a book that I ordered some time ago. It was marked "label printed" but never marked mailed. It arrived today. How do I let the powers that be lnow I received it? I don;t want to cheat the sender out of a credit.

Check your transactions archive. It should be in there. You can marke the book received from the Transaction Archive.

Is it going to be in your transaction archive if it was never marked mailed?!?!

Have you checked to see that you don't have another copy from someone else on the way to you?  If there was more than 1 copy in the system, it would have ordered another one when the 1st one wasn't marked as mailed. You do not have to give a credit to both people, only to the person who marked it mailed on time.  Check that first so you don't end up out 2 credits.  

It should show up in your transaction archive with an X as a cancelled transaction.  You should be able to mark it mailed there.  If not you can contact PBS and ask them to mark it mailed for you.

Mary described it really well.  Here's the Help Doc, too.

I received a book from a canceled transaction!

PM the sender.  They may have simply forgotten to mark it mailed (I've done it and so have a couple of my senders) and a gentle reminder may remind them, then you can mark it received in the regular way.