+1 catwings comments.

I made a mistake in a booking and promptly called them. I hadn't read the fine print on the cancellation policy of the particular hotel, but fortunately it was one that permitted a cancellation. The person was very nice to me over the phone, and immediately made the cancellation. All in one 3 minute phone call.

Another website that permits cancellations based on the hotel's policy is www.venere.com.

Romelle