That's a good suggestion, I hadn't considered that. I could charge a fixed fee for access to the relay server, and then if it weren't in the relay's cache it'd use your API key so you'd be dynamically billed without losing access to the cache. It'd be a fairly large change for a small audience though (how many people are willing to setup API keys etc?). You'd find that it'd be more expensive if you used it a lot and less expensive if you didn't - right now I'm giving people access to ~1.8x more credit than they're charged for (please don't abuse it! I'll notice with how few users I have, anyway).

Right now my focus is on why not a single user has converted from the demo, and yet I got four from posting it on producthunt without a demo. I think I might not make it clear enough when it's ended - it stops translating and tells you if you open the popup, but maybe people aren't noticing?