Hello, Is This Letter for the Demon Lord's Castle?

Nineteen years into his transmigration, Lin Gui barely survives in a crumbling post-apocalyptic city—until a simple act of kindness binds him to the "Messenger System." In a world where billions of demons roam and civilization has collapsed, delivering mail is the only path to power. His quest takes him from the sunken mermaid kingdom of Atlanna to the cloud-piercing angelic city of Eden, through forbidden academies and endless mazes. Along the way, he gathers an unforgettable party: elves, princesses, saints, mermaids, fox spirits, and more. His deliveries? A dragon's egg, a hero's lace dress…