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.
And, in the end,
the love you take is equal to the love you make.