No idea about the US but this very same case was tested in France and some part of Germany in the late 90s when some PayTVs (Sky or Canal+, can't remember) tried to cancel multiple subscriptions offered with an extremely aggressive pricing by some of their agents. Courts concluded that the signed agreements superseded the official pricing and they had to offer the service for the entire length of the original subscription.
Where I live, "meeting of the minds" is necessary for a contract. Written or not. In this case, that meeting didn't happen. Due to the bullshit generator employed by Air Canada.
So there was no contract but a consumed flight. The court has to retroactively figure out a reasonable contract in such cases. That Air Canada couldn't just apply the reduced rate once they learned of their wrong communication marks them as incredibly petty.
