I received a message from a member that a book that they marked mail actually can't be mailed.  She made a mistake.  

Message:  "...I'm sorry but I'm not able to mail the book. I had the wrong one to wrap and mail yesterday when I marked it as mailed. Sorry again. I hope you can just mark book not received so the credit can be reversed..."

I don't see anything that will allow me to on my end without waiting 4 weeks to mark "not received".  So am wondering, Is there something on their end that they can mark "CANCEL"?  So that I can get my credit back?

She needs to contact PBS and have them cancel the transaction, explaining what happened. If she doesn't you can ask them, but they usually requires the sender to initiate a cancel, though since you have her message it might help. Otherwise, you would have to wait for it to be declared lost in the mail and get your credit back. If it's WL you'd also go back on at your original position

Exactly what Linda said...the sender must contact PBS to cancel the transaction. Hope she cooperates!

I emailed the mailer and she contacted PBS.  I have my credit back!  thank you all for your help!