Does this rely on an external server (assuming so with the API key) and if so how taxing is it for you, is it just going to become too expensive one day and get shut down, or is operation trivial?

It's a really great idea, I had exactly this problem earlier with reddit (I saw a submission about the redbull stratos jump that had a comment I was interested in, but I couldn't relocate the submission...) and it's happened here before too.