This method is by far the best method. I used to use the customer service number on the Sirirus Renewal flyer they sent out, which I believe is their general customer service number, and it was legit like pulling teeth to get a discount. They were willing to let me cancel and lose me as a customer. Every year I think of SiriusXM as less and less worth the money due to the increasing commercials. But, it is better than my local FM stations.
I just called, told them that $350 for two radios was not worth it, and I would have to cancel. She offered me $99 for both, which is $112.14 including taxes per radio.
For anyone who calls multiple times a year for multiple car radios, I suggest combining them. I did this, so you just renew one car early, and they give you a service credit, and then you can renew both cars at the same time. I hate doing this annually, never mind more than once a year. Good luck everyone!