Jumyou wo Kaitotte Moratta. Ichinen ni Tsuki, Ichimanen de.

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)