Three Days of Happiness: The Complete Omnibus by Shouichi Taguchi, Stephen Paul, Madeleine Jose, Sugaru Miaki

Kusunoki used to believe he was destined for great things. But now, approaching twenty years old, he’s a mediocre college student with no ambition, dreams, or money. Nearing rock bottom, he learns of a mysterious shop where one can sell the time they have left on Earth…and decides to cash out all but three months. To his surprise, his remaining time comes with a girl, Miyagi, who’s assigned to monitor him until his death. With her at his side, Kusunoki sets off on a mission to tie up loose ends and―maybe, accidentally, and with a lot of help―discover what it is that gives our lives value.