On my public shelf I say that all books are 3 for 1.  Thus far I have had one person go with this deal, and when they did they ordered a book and sent me a PM about the freebies they wanted. 

Someone else just ordered 3 books from my shelf.  Should I assume they are doing the 3 for 1 deal?  In that case do I refund them the 2 credits once they mark the book received?  Do I tell them that they are 3 for 1? 

If they ordered from the order more option, then they might have not seen your bookshelf header. 

Personally, I'd PM and make the offer.  I would be surprised that anyone would expect the offer without sending a PM to you.  That is not standard.

if  they don't PM me, and order the books from my shelf, I just go ahead and send them like I normally would. I have a deal posted in my profile so anyone can see it.

When someone does that, and I am offering a specific deal, I usually let them know. I offer to make deals in my header, but every once in a while, I have a specific deal that I am doing, and if that is the case, I figure its like a nice surprise for them.