I believe that if you let wordpress users to check out how they could customize their design before buying “custom design”, the sales of this upgrade will definitely increase. Of course there is a refund, however it is not a customer- friendly and easy way to try this upgrade.
It is just like you did with themes. Firstly you try it on your blog and only then you can change it or buy it. I think it could be the same with this “custom design” upgrade. Firstly you could customize your design and then if you want to make the changes visible, you could buy the upgrade.