A member requested a French novel from me that was used as a textbook in a college course.  I accepted, chose the latest mail by date, and PMed the member letting her know the condition and asking to confirm that she still wants the book.  It's been 3 days, the mail by date is approaching, and I haven't heard back.  I'm assuming that I have to cancel the request if I don't hear back?

This grinds my gears because in the header of my bookshelf, I state that the French books are being posted as textbooks and I need confirmation before sending them per PBS guidelines. 

Yes, you will have to cancel or let it time out, you cannot send the book without hearing from them. Luckily it sounds like a book that would not have many (if any) other copies in the system.

She may have never looked at your bookshelf header.  Some people don't have computer access everyday. 

I would send another PM and tell her you are going to cancel the request if she doesn't respond. 

Also keep in mind that you have 2 days after the mail by date to mark it mailed.

I'd just let it time out rather than canceling.  I see no reason to cancel before the system does, though you might have to hurry and mark it mailed after they PM you even if you can't mail for a day or so after.

If the book was postable, I would pm a date several days out and send if the requester hadn't responded.  But under the textbook guidelines you must receive approval...wish there was a way to have this transaction time-out on her account instead of yours.  Hopefully she will contact you over the weekend