I was in Ottawa (YOW) last month. It was amusing hearing the US official say to me "welcome back to the United States" when I had still not left the Ottawa airport.

Speaking of Duty Free, flying to Ottawa I connected in Detroit. The flight to YOW was cancelled and we all had to overnight in Detroit. Those who had gone duty free shopping in Detroit were told they would have to return everything to the store for a refund and then, if they still wanted it, purchase it again tomorrow.