Relay user here. This is probably obvious to most but I feel it should still be stated because people don't read everything. The vibe I got from DBrady's markups was that he plans to make it a prepaid subscription thing. You'll be able to see how many of your API calls you've used and will have the option to upgrade your plan at any time

Here's a markup https://i.imgur.com/lIbBPJo.png