I use the XM "family plan" for all the family receivers. Saves quite a bit in monthly fees. You might want to consider that for your two cars. I even share with family in Texas. I pay for as many years in advance as possible so I don't have monthly billing.

I'm not surprised to find it's hard to stop service. Anything you authorize to be paid automatically creates a temptation for the supplier to hang on to you as long as possible.