From the Super Mario Wiki
The Toadmaster General is a Toad that appears in Paper Mario: Color Splash. He owns the post office in Port Prisma. At the very beginning of the game, he is mailed to Princess Peach colorless, folded up, and stamped with the logo of Prism Island. This leads Mario, Peach, and Toad to travel to Port Prisma to find out who mailed him to them. When Mario receives the Paint Hammer, he is able to be revived and will go back to the post office. When Mario speaks to him in the post office for the first time, he will fix the crooked sign out front, allowing Mario to get a Mini Paint Star.
The Toadmaster General stays in the post office for the rest of the game, where he will give Mario any new mail he has received. After Mario rescues the Yellow Big Paint Star from Iggy Koopa at The Golden Coliseum, it reveals that a group of Slurp Guys drained the Toadmaster General of color so that Kamek could later mail him to Peach, as a way for Bowser to lure her to him.