Ask HN: What are you using to bill one-off online calls/meetings?

I receive a small, but non-trivial amount of emails that request my time (via a call or otherwise). When appropriate I reply (or chat) for free, but I can't do this for everything.

Instead of just saying "no", I'd prefer to have an option where I can tell the person to schedule a paid chat with me.

I've seen the likes of Code Mentor, but are there any services that cater for this in a more general-purpose way?