The Contact Support button is located in the upper right corner. http://docs.sellfy.com/ They usually respond within a few hours.
You can submit a ticket, call, or do the live chat. http://support.clips4sale.com/
If you are unable to rent a video because it states that the video is not available in your country, I can look into that. But, for some reason, YouTube doesn't allow Germany to rent USA videos.
As for their customer service, you really can't contact them, but they have lots of helpful documentation at https://support.google.com/youtube/?hl=en#topic=4355266