I think there is no problem to drive US rental car in Canada.I had an accident in New Jersey while coming back to my home in Toronto.My insurance company arranged a rental car from Enterprise. At the Border, I just them the truth. He cleared me in 5 minutes. He just told me that you should return the rental car to US within 60 days. When I returned the rental car in Toronto after a month, Enterprise arranged the US Plated car to send to Airport or Niagara. Enterprise Rental Car had no issues to send the car back to US.The Manager told me that it is their normal business If you make a deal with Rental Company, most of the time you can avoid Drop Off charges.