I tried to message the creator https://www.deviantart.com/danioc but no reply and he seems inactive for years. My design would be a big benefit for your Window Blinds software so I'd like to sell it to your users.
Dani is active, some of his design can be seen in Curtains. Not sure why he does not respond to your request. https://www.wincustomize.com/users/20239/danioc
As for selling on Windowcustomize.com, yes, as what Tom (LightStar) mentioned, only Master Skinner level will be allow to sell their work.
Stardock Community Assistant