回複：去加拿大LANDING，在收到瘋爺卡之前可以回美國嗎？

It's OK because the card will be mailed to you. It's different for someone who already landed. In that case, he has to pick it up by himself. But when you land, be sure to tell them you will stay until you get the card.

As to the car, it is OK if you do not drive it to Canada when you land. If you drive your own car and you want to import later, you could have some problem.