Unsouya no Ossan ga Nazeka Fukugyou de Zettai Muteki Kenshi wo Tsutomeru Koto ni: Saenai Jinsei wo Okutteta Ore ga Maou Toubatsu no Kirifuda ni?

Jack, a lackluster deliveryman, is living a boring life when he is suddenly summoned to the royal capital. There, he is startled to learn he will receive the tile of "One Sword," which is bestowed on the truly strongest swordsman. It's the epic tale of a boring middle-aged man with hidden power!

(Source: Akitashoten, translated)