You might be able to return the digi and swap it. Personally I don’t think their website is clear enough considering the audience.
However, do you actually need one of their boards? The sound quality from the Pi’s onboard line-out is actually pretty good these days in my opinion. It was initially horrible but no longer deserves its bad reputation. HifiBerry et al. don’t like to mention that, for obvious reasons! However, do note that the onboard 3.5mm audio port is not really a headphone port, it’s a line-out and ideally should be plugged into an amp (a cheap one will do).