I went through the same shit a few years ago trying to cancel Sirius XM. When I spoke to a rep, I told her right off the bat I sold the car. I need to cancel the service. She then proceeds to keep hard selling me to keep the service by offering discounts and stuff. I finally asked her why is it so difficult to understand that I do not want to pay for service for someone elses car?
AMEX is the best for handling chargebacks. Not a ton of questions, just a simple did you authorize the charge, and I know it will be taken care of.