I don't think the OP did anything wrong. She wanted to buy something. She put it in her "cart" and checked out. She paid what they asked for it. On the same token, I don't think legally they have to honour it but it's certainly worth a shot. I respect the companies that take ownership for their mistakes a lot more than those that don't.
It might not be a mistake. As someone else said, companies often offer things for free and you just have to pay the S&H. I do that with business cards online all the time. I've never paid for a single one and I use a lot of them. Sometimes companies do it so that while you're there getting your free whatever, you might look around and buy something else.
If they cash that cheque, I don't think you can do much about it. However, you could always stop payment if you realize your error. Regardless, it's your mistake and you should have to take ownership for it just like the company when they make mistakes.