Far as I know, which is not a lot, Canada and the U.S. have an open-skies agreement, part of the NAFTA. (North American Free Trade Agreement). Why Mexico, as a NAFTA partner is not included, I don't know. By their own choice, I understand.
Westjet's transborder operations are all scheduled service. No bi-lateral agreement is necessary. Just pick your destination and fly there as many time a day as you like, local Airport Authority beauracracy notwithstanding.
Take care.