Just leave it, if I remember correctly, downgrading service doesn't actually happen until the end of your billing period, no refund for the rest of the billing period you won't be using it. Turning the service back on from Standby takes about 10 minutes, and your billing is prorated from the day you turn it on.
As for using it in the car, get a 100W car plug to plug it into. I would stay away from suction-cup mounts if you plan to drive down the road with it on the roof. Too many stories of people finding them on the side of the highway. On my vehicles, I place it in the sunroof and close the shade, it works just fine.