If the other person doesn't mark the book received then that means I'm out of a credit if I didn't use PBS postage, is that correct?  I have tracking through the post office, and it says it was delivered 1 week ago, but they haven't marked it received yet.

You can contact PBS with the tracking number after the Lost date. PBS will then check to see if the member has logged on recently or not. If the account appears inactive, they will often move the credit.

Please note that the member receives two reminder emails from PBS in the week before the lost date.

Try sending a PM to the receiver including the tracking information and ask for them to mark the book as received. It may prompt them to do so.

I have found when I do as Mary suggested that they always quickly get marked 'received'.  Those tracking numbers that are 'evidence' of delivery (even when credit insurance is not purchased from PBS) seem to give people a 'kick in the pants' to do as rules state. (I know there are sometimes delivery errors by PO or 'delivery' is made to a PO box or held mail when the recipient hasn't actually gotten the package yet.)

I sent the tracking # on 9/2 right after I sent the book.  I then sent a message on 9/10 making sure they received the book ok after seeing that it was delivered on 9/5 but haven't heard anything yet.  Thanks everyone!

actually people not marking books received seems to be a very common thing here. It's happened to me many times.

I have only been active 1 year even though it says I have been on here since 2011.  Luckily this is the first time this has happened to me.  There is still a lot of time to mark received so everything could be fine.

This is covered in PBS guidelines: 

If you sent the book with printed postage/tracking purchased elsewhere and it has been recorded as delivered by USPS but hasn't been marked received yet:

• First, contact the requestor from the active transaction (in the Books I've Mailed tab in My Account), or from the "lost" transaction in your Transaction Archive if the book has been declared Lost in the Mail by PBS.
• If the requestor does not respond to your PM within five days, contact us and we will look into it.
• We cannot grant credit for postage or tracking purchased outside PBS, but we can check if this requestor has become inactive.
• If this requestor is inactive, your book should be marked received by the system.

If your book does not arrive by the 26th day (the 35th day if the sender or requestor is outside the contiguous US) after you marked it mailed, it will be declared Lost in the Mail at PBS.  This does not mean that the book is actually lost; it may simply be delayed en route.  You can read more about Lost books in A book I sent is lost in the mail!.

Karen, I've had to ask PBS to check into the activity or inactivity of several members in the past who didn't mark books received nor responded to my PMs...and I've always gotten my credit.