I am assuming that an instant credit you receive is guaranteed even if the person does not mark it received before it goes lost in mail.  Is  this correct?   i ask this because I sent a package within instant credit and the tracking shows it delivered. This is really not big deal the person could be on vacation and will mark it received when she returns, but i just want to make sure i still have the credit.

Yes.  The fee covers the "insurance" to guarantee the credit.

What Emily said.

Also, it would be good to send a "Hope the book arrived... Please let me know if it didn't" PM to the requestor.  If you don't hear back in 5 days, let the Admin Team know.  They can check to see if the member is active.  You might be helping out another sender who mailed a book to her without PBS postage or DC.

That said, when I hold my mail for vacation, the tracking reflects it as delivered. Even though it is locked up in a bin at the post office.

But the PBS reminders go to email NOT your message acct on PBS so sending a PM could reach someone who is checking their PBS acct but not email regularly.  That is possible as I recently experienced--- my father died and I stayed with my mom for a month to help out. My daughter had her IPAD with us, but I could not check my email on it as IPADs are only set up for one email acct, though I could access PBS thru the internet.