Can a doctor's office legally charge a "no-show" fee?

Can a doctor's office legally charge a "no-show" fee? I made an appointment to have some bloodwork done but it wasn't like "ok we have an opening at 8" or anything...they basically said "we open at 7:30, make sure not to eat breakfast, and come by any morning this week" and I said ok. Well I didn't go, I never had the time to do it. So today I get the mail and there's a $50 charge for that. And I do remember signing the no-show policy agreement paperwork, which does outline the charge...but considering my appt wasn't with a DOCTOR, it was just for a nurse to take some blood...2 minutes of time, I don't think they can do that. I was under the impression that "just come on in" meant there wasn't an appt scheduled, I would have never screwed them over like that. I plan on calling Monday but i'm afraid they won't waive the charge. What do you think??