SiriusXM Calls twice daily everyday Columbia sc

I bought a new automobile and it comes with three months of free SiriusXM radio. A month before my trial period ended, SiriusXM constantly called me at least twice daily hounding me to sign up with their 6 month promotion for $30. I told them no several times and they still keep on calling me. I finally told them on this last call that I would pay $20, but not $30 for six months. They said they could give it to me for $25 for six months. They tried to get my credit card number, but after reading all the horror stories of SiriusXM withdrawing money from people's account and taking them to collections I decided to have them invoice me so I could pay by money order instead. Apparently I can cancel at anytime. Frankly I'm a little afraid that when this six month period is over, and I choose not to continue, will they put it on my credit report? With some people saying they are charged $27 for six months and I am charged $33, I am wondering what type of scam they are selling. Maybe I should just plug my phone into my radio for preferred music from (((competitor's an,e redacted))) instead of dealing with SiriusXM.