I did this a few years ago, I'm a British national, arrived in the USA via air using VWP, we did as CMaster suggested checked with our car hire company (Hertz) that going to Canada would be allowed, and ensured it was put on the rental agreement. The only real thing to add is when we came back in the USA they wanted to see our car hire rental agreement so make sure you keep that safe and available at the border crossing. I don't remember there being a fee in either direction, apart from the bridge toll when we entered Canada at Niagara Falls, and we found the border crossing pretty easy both w