As popularity of social couponing websites increases constantly, so does the competition. The good and usable design of the websites interface makes a huge difference between success and failure in the group buying market. In this paper usability evaluation is conducted on a relevant case study. The usability of an group buying platform was tested using two different methods: heuristic evaluation by experts and scenario testing by users. Results show that each method uncovers different types of problems that contribute to the improvement of the overall performance on the website.