I called XM just now and threw them a curve. I wanted to take another car with an existing XM account (which was at full price, paid quarterly) and then put that car under my new account at the $77+6.99/mo (2nd acct.). I figured the battle would be confusing and immense.... I was wrong. It only took two times to say how the account would be set up and they got it right.
They tried to credit card me... they were denied
They tried to tell me the bill would be ~$164+6.99/mo (second car). I said that I want the $77 deal. She asked if I had a code, I did not. A few seconds pass and she says ok. And also told me that the $77 deal is a one time thing. (uh huh

)
$77 for the first car
$6.99/mo for the second car (~$84/yr forever when with the primary car)
minus balance on the first car's existing account which was paid quarterly
I could have tried to get the $77 twice on the same account, but I wasn't into dealing with a hassle today.
hassle free>$7
mailing me a bill (nothing was said to me about an extra paper/mailing fee)
Owe $136.95