Do you have more information on that? Did you do a deal with Appsumo before? What are usually the terms for such a deal?
As I know they have about ~1M users. Once your product is verified almost all of them receive a newsletter about your product. Your product is sold by the fixed price forever. But they take a big % (40%+) for that deal. I heard from some projects which have been used AppSumo and that the impact is really great. Thousands of new users and sales.