It's my opinion, and JUST my opinion, that you've built up enough that it's time to stop doing it for free. Unless it's for a good cause, or for a really prominent publication. And, frankly, it sounds a bit like this most recent invitation is prominent, but maybe not...? What would you get for the article in terms of leveraging it for future paid work?
But, it's been my experience (please don't flame me for this, folks, I realize everyone's different!) that if you don't charge you don't get taken seriously. And people reviewing your past work will know that at first some free stuff is fine, but, after a short time you ought to be getting *something* if you're trying to make a living at it.