You won't get charged a foreign transaction fee on AirCanada.com, as long as you are on the US site. The first time you open AC.com, it will ask you you're country of residence, and if you put United States, you won't be charged (I've paid with Bofa, Chase, and RBC Bank (USA) cards, and never been charged this fee).

If you are on the "Canadian edition" of AC.com, then yes, you will likely incur this fee.