If you call the customer service number on the USA web page from Canada, the phone system detects you are calling from Canada and you get automatically redirected to Canada's customer service.
You should be able call the Sirius customer service in the USA directly using 1-888-601-6297.. I've been using this number to get new receivers activated on my US account... I haven't used it for about a year, but I believe it still works.
I've been paying for Sirius with a Canadian Visa card for six years now. Actually, in Visa's rules for USA merchants it specifically states that they may not refuse to honor foreign issued Visa cards. Sirius USA has a right to refuse you service if they believe you live in Canada, but technically they can't refuse a subscription simply because the card was issued in Canada.
One thing to consider is that some states have a sales tax on satellite radio and SIRIUS will add it to your bill and remit it to the state. This will increase your monthly cost.
FWIW, Arizona is one state that doesn't charge any tax. So, if you subscribe through an Arizona address, you won't pay any tax at all on your subscription. I was lucky enough to renew my 2yr subscription when the dollar was near par. If I average out the 2 years, my US subscription is only costing me just over 11 bucks a month and I don't pay any tax at all for the service.. No state tax, no PST, no GST