Traveling Through the Post-Apocalyptic World? but I Have a Survival System!

Please Help Me Find This Book

I don't remember much of the plot, but I vaguely recall it was an infinite-flow story with two male leads. Everyone else paid with money, while the protagonist paid with his life. If I remember correctly, the first instance was something like an experimental apocalypse setting, or at least one of those elements was in it. In the second instance, the protagonist was a white-tailed merperson.