Tegami Bachi 

Gauche Suede is on his last delivery before a big promotion. In the outskirts of Yodaka, the darkest area of Amberground, Gauche is surprised to find that the package is a young boy named Lag Seeing. Lag had been traumatized by his mother's abduction and is due to be delivered to his aunt. In this remote area rife with Gaichuu, Lag and Gauche face a dangerous journey that inspires Lag to become a Letter Bee. (Source: VIZ Media) Note: Includes one extra chapter.