I Lie Low and Play Weak After the Supernatural Arrives

[Slow burn + steady progression + no dumbed-down characters + lighthearted humor + clear-headed protagonist + not mindless power fantasy + a little cautious + occasional brutal takedowns] The supernatural has returned, and ghost fog blankets the world. Humanity survives only in twelve cities. Xie Xun becomes a courier for the Yin-Yang Post Station, traveling through the terrifying ghost fog to deliver letters imbued with mysterious power. To keep himself alive, this courier not only puts on a frail, unremarkable facade at all times, but even when he steps outside, he carries more than ten pounds of talismans and magical artifacts! I only want to lie low and grow stronger in the background. But somehow, every time, I end up making a huge commotion and attracting everyone's attention. Heavens! I just want to be an ordinary person, staying out of sight. Why are you doing this to me!