Credit card or prepaid card: Problem with a purchase shown on your statement

I had a " locksmith '' come out to a hotel I was at to make a new keyfob as mine had been damaged. The locksmith quoted me {$280.00} written on a work order form. Although it seemed a little high I signed it anyway as they took a long time to arrive and I was late for a meeting. I then found my credit card was charged {$780.00}, not {$280.00}. I looked at the work order and they had written in additional items and changed the 2 to a 7, after I had signed and authorized the work. Attempts to contact the number on the work order went to a dead line. After further research I found out it was a fraudulent locksmith service that utilizes search engine techniques to direct searches for a local locksmith to a fake website that appears local, but the call goes to a central dispatch and an unlicensed locksmith performs the work at an enormous price. I filed a complaint with our state government and they confirmed it is a fraudulent service and sent the issue to the state attorney general for potential prosecution. I disputed the credit card charge and provided this information but they found in the merchant 's favor because there is a receipt with my signature on it.