Is there any way in which PBS could somehow reward people who request and/or send multiple books in a single transaction? 
 

Don't wait for PBS. Do it on your own.

1.  In the Book Bazaar your can take adevantage of multiple book deals

2.  When you use Book Browser to find a book, open all the choices you want by right clicking on the title and open in a separate tab. Once on the new tab, order the book then go to the member's bookshelf to see more he or she has. Find all you want and send a PM to that member that you'd be interested in buying x books for y credits. some will, some won't. IMPORTANT!! - make sure you back out to the original order page and "Cancel Order" instead of just closing the tab or you'll end up with ordered books at 1 book for 1 credit each.

3.  When someone orders books from you, PM them before you send out the books if you want to give them a deal for more than multiple credits.