I called up before my reservation in Calgary and they told me there is no such thing. I had to spend some time talking on the phone with a rep who advised me to bring proof of USAA membership ( just printed out my email from there, that was enough) and a copy of where it states underage is waived from the website. He told me to tell them I had spoken to the manager if any problems came up, which I did but didn't end up having to show them any of the papers...
basically I'd advise you making the reservation and then calling up to CONFIRM that all your details are correct and dealing with it before you get to them. AND don't ASK if it's covered in Canada. Just let them know according to USAA it is.
BTW that will only waive underage fee for the MAIN driver IME.
