When books are added to someone’s cart they as the buyer can see that they are all from the same shop. Sellers don’t get that view that one user has multiple books in their cart. This causes problems when offering discounts on books that are in a cart. If you offer a discount it only impacts that one book and treats it as a single sale voiding any other discounts they might be getting for buying multiple books from the same shop. Giving the seller that view would allow them to offer a discount on the bundle instead of the individual book.