There never was duty, only taxes.
You missed the entire decades long arguement about softwood lumber? And steel?
There was effectively no duty on imports MADE in the USA (and Mexico) but imports from the USA but manufactured elsewhere were, and are, dutiable.
Canada has Free Trade Agreements with more than 100 countries which complicates things.
But if your widget, purchased from a US seller was made in China, it is subject to duty.
Couriers charge a BS brokerage fee
The $25 or more "brokerage fee" covers the cost of the paperwork for CBSA. And of course, a profit for the company.
That the GSP only charges ~$5 for this is one of the very very few benefits of that Seller Protection program that otherwise buyer-unfriendly option.