That seems fair. So, example:
I fly YQR-ORD-BUF, do some touring, visiting and shopping, and then drive my rental car to meetings in Toronto. If I've been in the US 48 hours or more, I pay no tax on the car; if I have been in the US less than 48 hours, I pay Ontario HST on $200 per week I intend to be in Canada. So, for a few days in Toronto (less than a week) and then back to Buffalo, I'd pay $26 in tax (13% of $200).
That seems more than fair. That's the tax I'd pay on rentals in Toronto, approximately.
And if this seems like a crazy example, sometimes it is cheaper for me to fly to BUF than to YYZ, and some of my Toronto-area meetings are in Niagara-on-the-Lake, which is a lot closer to BUF than it is to YYZ.