https://estimatefee.com/n/$CONFSwhere $CONFS can anything from 2 to 1008. We directly proxy the results from bitcoin core using `estimatesmartfee` (with a 30 second cache), so the value is in bitcoin per 1000 bytes. If you're using this, make sure to put a fallback. If it works, it'll be accurate. But don't expect five 9s of reliability -- because, really, I really only made the API to look cool.