The US no longer issues transit visas. What nationality is the PAX from? Some countries are not required to apply for a Visa in advance.

I believe you will need a visa. Some airports have International segregations points for transit, which point you can get by without a visa. The problem is (you already know this) many US <-> CAN flights you actually clear customs in CAN so that the flight can be treated as a domestic flight coming into the US. If the passenger needs to clear US customs for transit (e.g.; YHZ to IAH ) they will need a visa. As such, you may be able to go one direction and not the other for your transit. The airline would be able to confirm this for you.

You can book the ticket without a visa, you will not be able to board a flight without it.