Simplest, lowest-level packet relay service?

I've used Photon and some other high-level networking packages, and they just don't suit me. I'm happiest working at a low level, doing my own state syncing, using something like the Hazel library.

Only trouble is, with Hazel I need to run my own server. That's fine for prototyping but I worry about scaling it up. If the game suddenly catches on, or we decide we need regional servers to make sure folks in Japan are getting just as good service as those in my area, I don't want to be stuck without an easy solution.

So. What's the best service for just helping me get my own data packets from client A to client B, as quickly and simply as possible? And ideally, cheap too?