Mikkakan no Koufuku

Three Days of Happiness is a novel by Sugaru Miaki, also known as Fafoo. It’s a rewritten version of the story originally posted to 2ch. If you haven’t read the original story, the premise revolves around a shop where one can sell away the remaining years of their life, and the consequences of doing so. Also, I Say A Little Prayer is a short related story which was posted along with the novel’s release. It spoils some things about the main story, but nothing major. (Source: Novel Updates)