10 minutes got me a quick refund on my service that was auto-renewed.
1. Called 1-888-539-7474
2. Press 1 (English) then 3 (Accounts), then 2 (Account Inquiries).
3. Told the guy on the other end. "My account was just auto-renewed. I would like to use Promo Code SXM60 to pay less for my current package."
4. He asked what plan this was for.
5. I told him "SiriusXM Select plan for 1 year."
6. He refunded the difference on my card.
7. He told me to call back just before my renewal date next year and do this again.