Kim,

I would call them and explaine the situation to them. If they are ANY kind of decent human beings, they'll pay you. If not, then it's an expensive lesson. My hunch is they KNOW there was a mistake made and said nothing hoping you wouldn't notice. How do I know this?? In all honesty, because I USED to be like that. I'm not anymore.

Don't EVER say you're stupid. You are very smart and talented.