I honestly have no idea but, this is the response I got when I asked a Canadian acquaintance.  I doubt this will answer your question but, here is what he said.
Originally posted by Patrick:
Yes... although I suspect that post might be old, as the situation has changed somewhat over the years.
Cross-border shopping/shipping (from the US to Canada) is something that I've dealt with for decades, always on the receiving end. (I've never shipped anything across the border to the States.
Yes, it used to be the case that using the postal service was always the best way to minimize duty charges and brokerage fees. The postal service didn't charge a brokerage fee, and duty wouldn't be collected at all if the value of the shipment was less than 20 dollars. That may or may not be the same situation when the shipment is going the other way (from Canada to the US).
In the last few years, another option has become available, where the vendor themself collects the duty prior to shipping (the amount is added to the original bill), and the shipping company is then not involved in collecting the duty... therefore no outrageous brokerage fees. So in this situation, it's fine if a private carrier (as opposed to the postal service) is used for shipping. RockAuto now ships in this manner, and it's worked out great.
Of course, the whole situation has now become a lot more complicated with Donald Trump wishing to apply variable tariffs on various items... and it seems to change week to week. So... it's difficult to give a definitive answer on the best way to now ship across the border with the way things currently stand.
Rams