Okay, have investigated, and this was a failure on my part to grok the shop interface.
However, in the interests of giving good feedback: searching for 'Brink' in the catalogue shows Book 1 listed as 'Digital Edition', book 2 listed same, and the Book 1 & 2 bundle listed as 'Paperback'. If you click on Book 2, Digital Edition is selected, and Paperback is greyed out, which to me reads as unselectable, even though I now realise it is actually selectable. Might I suggest that you show 'Digital Edition' and 'Paperback' as separate results in the search interface, and put a little bit of black into the additional format selectors. 
Many thanks for the feedback. This very point has been the source of much debate in the web team and the webshop has been through multiple iterations. We briefly considered having separate results for digital and physical but, in practice, so many search results looked even more confusing.
Regarding the first point you make, any listing that has a physical copy includes a note like this:

We've been pressing for physical media to appear first in searches but there's database issues with out-of-print books that will take a very long time to resolve. It's something we're aware of but there's no easy solution that covers all the bases at the moment.