The range of books on offer changes regularly in the shop, in order to always be able to offer an attractive range. When you choose books in the webshop you will see a number of book points at the top. The books shown are all worth a certain number of points. This way you can choose a number of books yourself until your books run out of points.
Does the book of your choice have a lower value than the displayed book points? First check whether there is a book category with a lower point budget. Perhaps the book of your choice is also included.