You should be able to check on your detailed receipt of each charge. I would call the office you rented from and just let them know you're quoted amount.
Michael, I am not sure how rental car companies charge for satellite radio. However, I have it in my personal vehicle. They, too, told me it was $X per month, but the bill is higher. This is due to fees. They also charge a tax. Therefore, the charge does not reflect what "They told you". Hope this helps.
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