I printed a label and marked it as send then realized I didn't have the book.  Someone had already requested it and it was sent.  I can't figure out why it was on my shelf when it had already been picked.

How should I proceed?

I would PM the member explaining what happened and also refund their credit immediately. Ultimately the book will be marked lost in the mail.

Since you marked it mailed: I would buddy them a credit and ask them to mark it received. They can then order the book again. If you refund their credit and they dont' mark it recieved-they will end up getting 2 credits.  Marking it received releases their credit from escrow and you are post left in the same position. They just have to reorder the book.

Unless it's a WL book-then I would contact PBS and ask them to cancel it and put them back in line at #1. 

Actually Merrill, you should just contact the site owners to cancel it for you--they'll deal with the credit and reordering issue (use the CONTACT US link at the bottom right hand corner of the page to send them a feedback email.)

And, okay, I'll say it--this is why you should not even accept an order, let alone mark it mailed, without having the book in hand and double checking to make sure it's postable.

What Diane said.

Please, do not mark you books as mailed until you actually mail them, OR at least have them in your hand/car/purse ready to be mailed once you stop at the PO.

Marking a book mailed before it's not or before you even check to see if you still have it to send is totally not cool.