Short answer - HST only applies to the $23 YOW airport improvement fee (@13% = $2.99). The 5% GST applies to the rest of the flight charges.

Here's a clear breakdown of the rules: https://www.flyporter.com/en/book-tr...-taxes-charges . It's from porter airlines but the rules are clearly explained. For Canada -> US transborder flights, HST (13% in Ontario) only applies to the airport improvement fees, but the 5% GST applies to the base fare, Nav and Surcharges, and ATSC charges.

As for the official government source that thetaxman linked to above, it clearly states that international air travel is zero-rated, "except to the continental United States and the islands of St. Pierre and Miquelon (this is defined as the "taxation area"). It gives the following example: A person purchases a return flight from Halifax, N.S. to Boston. Both flights are made in a non-participating province (therefore only GST applies, not HST) because there is a stopover outside Canada.