Towing company overcharged

Hello,

So I had to get my truck and trailer towed. They told the driver it was going to be 400 dollars. I had to pay over the phone and gave them my credit card, but I noticed they took 535 instead so I called them the next day and asked why it wasn't 400. They said they had to do extra work to tow it but never told me that they were going to charge me 535 when I was on the phone paying them. So I disputed the charges. They sent a letter weeks later saying they were going to take me to court and put a lien on my truck if I didn't pay them. And now instead of saying it was for the extra work they did, she said it was 250 for the truck and 250 for the trailer. I called and let them know I would be willing to pay them what we agreed upon, which was the 400. They said they don't want the 400. What should I do?