Hi @poweiharn , welcome to the community!
It’s not possible to get a static IP for serverless functions with any host that I’m aware of. that’s a bit antithetical to how you’re supposed to think of them, since they you aren’t supposed to worry about where they are hosted, or anything to do with the server they are running on. I’m not familiar with Line Pay, but I do not that lots of other payment processes will allow you to make payments from dynamic IP’s. Sorry I don’t have a great solution for you, but if your only option is to get a static IP then maybe a cheap droplet from digital ocean would be a better fit for the backend.