Okay since this is a bit messy, let's organize it like this. There are 4 albums, let's just call them House, Trance, Orchestra and Euromix. If you order one or some of the CD's separately, do the +1 thingy for each of the CD's you're buying. If you buy the box, you're basically buying all 4 of them, so put +1 for all four different albums.
So now VRGRK, miki_beland and kinix ordered the box, which makes purchase count 3 for every single one of the albums. I will put that here, and then post my own reply to show everyone how to count
House: 3
Trance: 3
Orchestra: 3
Euromix: 3