Customs question: US citizen flying out of Canada to Japan

I'm driving from US to Canada, to fly from Toronto to Tokyo. How do I handle customs when I return? Do I declare everything in Canada? And then again at the US border? Would I potentially pay duties upon entering Canada and then again when I cross into the US? It's never been an issue for past trips, but I anticipate purchasing more than normal this time. Thanks!

* Edited for clarity