Groupon warning

Hello all, this is more a warning than a question, but if you had same experience that would be helpful. I purchased (stupidly) 2 x 2 groupons for a restaurant at 48€. A month before it expired, I tried to make a reservation at the restaurant but was informed that they were full and could not take any more reservations so that I needed to contact Groupon to get refunded. Well, they are "refunding" me MINUS 15%!! I do not call that a refund. In their terms and conditions they say that if the customer cancels after a certain time, they will impose an administrative fee, but it was not AT ALL clear that this is also the case if the merchant FORCES us to cancel! I tried calling and complaining but got the regular excuse that it is in their terms and conditions. So be careful when you decide to buy a Groupon, especially if it's a large amount since the establishment can just tell you they can't take you and you're stuck paying the "admin fee".