You are probably referring to the auto-emails reminding you to mark your book mailed. The sending process is not complete (and you will not get credit for sending the book) unless a book is marked as "mailed" before the deadline to do so.

• The deadline for each transaction appears on the transaction on your account page, in red.
• If you get an email from PBS asking you to mail a book you have already mailed:
  • that means that you have not marked the book mailed.
• You should log in as soon as possible to mark the book mailed by clicking "Book Has Been Mailed" on the transaction before it cancels!

If you do not mark the book mailed before the deadline, the transaction will be canceled and you will not receive credit from the system. Canceled transactions cannot be reactivated.

Related Links:

Swapping Deadlines
How to send a book
My request was canceled but I already mailed the book!
My request was canceled--I didn't mail the book yet