It sounds like you may have committed fraud, although their calculation for damages is questionable. Flexible tickets are generally priced higher than a normal or saver ticket, therefore you have found an unethical way to leverage some flexibility. It would be prudent to determine how close to departure some of the trips have 'cancelled' and if any have cancelled on the day of departure or through no show?
You probably should contact them in writing with a good explanation or perhaps contact a lawyer and spend a few hundred dollars for them to draft an appropriate letter for you.