I ordered a book from a member and they didn't mark the book mailed on time. Yet, I received the book. How can I give them their credit or am I right in thinking that because they didn't mark it mailed on time that they lose that credit?

If it were me, I'd mark as received and let them have the credit.

There's a Help Document for this situation.  Basically, unless you received a second copy of the book (due to error by 1st mailer) you need to mark the book received from your Transaction Archive.  Also check and make sure the book was not placed back on your wishlist, or the system may offer you a second copy and further complicate the situation.

Was the book mailed late, or merely not marked mailed?  Late mailing is a different issue.

Do check that you are not getting two copies of the book.  The older order will be moved to your Transaction Archive.  And new "rolled over" order will be on your My Account main page.  Or the book may be added back to your Wish List.  Please check those places first.

Then you can go to your Transaction Archive, on your pull down My Account menu or on your My Account main page.  The old order will be there. you can mark it received, which will deduct a credit from you and give it to the sender.

Please be very careful in the TA.  This has cancelled and lost books.  Marking any book there will deduct a credit.  Please double check the return address.