Is it illegal to charge a convenience fee?

So, is it legal to charge a convenience fee? That's a question I've actually wondered myself, especially after that time I almost booked a plane ticket and they slapped a ten-dollar "convenience fee" on top! Ten bucks! For what? Clicking a button? Anyway, apparently it is legal, as long as they're upfront about it. I mean, really upfront. Not buried in tiny print at the bottom of a 20-page terms and conditions agreement, you know?

The way I understand it, it's all about disclosure. They have to tell you before you hit that "buy" button. Think about those concert tickets – I saw a band I loved last summer, and their ticket site was brutal for fees. But, and this is a big but, they warned me. So, grumbling, I paid.

And here's the other important part: you have to have a choice. If they only offer that payment method with a fee, it's a scam. Pure and simple. But if you can also pay cash or use a regular credit card without extra charges, then it’s generally okay, legally speaking. Seems unfair sometimes, doesn't it? Like, why should I pay extra just because I'm using a credit card? It's a legitimate question, I think. But that’s the law, as far as I understand it. Still makes me a little mad though. Maybe I'm just a grump!