So I recently asked Brandon on Reddit about how Hoid sent the letters to Frost and vice versa during stormlight. He got back to me with this " He's not using it, but there is active trade between several of the planets (and a version of a postal service.)". So that had me wondering what other kind of parallels there were in the cognitive realm. Like how huge is trade in between realms, I assume pretty large. Here are the messages.