Rental companies will usually charge the tolls in your rental, plus a processing fee.
Before entering ON-407, there are big signs saying "NON-ONTARIO PLATES WILL BE CHARGED", but I don't know to which extent. I wouldn't believe they have agreements with every single state in America, maybe just Québec, New York, Pennsylvania, etc.
Doug has driven on the A25 bridge with his NY plates and I think he never got billed.
I assume that with temporary tags, you get to drive for free. It's only temporary anyway. And in QC, those tags must be stuck inside the car on the rear windshield anyway.