This is the scoop. I have ordered, over the years, many $1000's of stuff from the USofA. I will mention, as I am sure many will note, it was a lot more fun before the knee-jerk reaction to 9-11. For instance, Brownells will now not even send you the screws to hold a base on your rifle unless you have a permit. But I digress.
If you buy from someplace that ships UPS (i.e. courier) you will get dinged with brokerage fees, duty and taxes. Usually. If the dollar amount is small enough, and the items fall in the right category, you may not get whacked as bad. Just because the items are made in the USA does not mean they are duty free, that usually means there will be duty to pay, unless the category is one where there is no manufacturer in Canada, and then they may let you off the hook a little.
If you can talk your supplier into shipping USPS (United Stated Postal Service) you will save yourself major hassle. Yes, you may still pay duty and taxes, but the stuff will be delivered to your door and Canada Post charges $5for brokerage fees, unlike UPS which usually starts at $25 US.
So, pick a good supplier, hopefully with a product that can be shipped to us terrorists here in Canada, and then talk nice and see if they will take the time to ship USPS, it is worth a little extra shipping to do so.