Duty is an import tax. It does not matter where it comes from unless Canada has different taxes from different countries. I would find it difficult to believe that they are giving anyone a better rate the the USA considering the huge volume of trade between the two countries going both ways.
You have to be careful how you mitigate those fees seeing they are based on the price paid for the product.
When you ship overseas (or cross the border) you have to file a tax form as the seller. The only way to get the duty cost down is to put a smaller figure in the box. That is illegal. In fact, just asking a seller to that in so many words is illegal. If you ask a seller to do that on eBay you may get a letter from eBay that you may be guilty of conspiring to file a false tax report.
One thing I leaned from watching other people make the mistake, never mess with any taxing agency. They have unlimited funds to go after you if they decide to.
I am just guessing but I think the bulk of the sales are in the USA so I am not sure how big of a problem that can be. Most people know that they have to pay import fees.
All that said, if you are contacted by a buyer trying to get the fees reduced, tell them to buy the product flat or rolled in a tube and have it framed or stretched locally. That will reduce both the shipping and the important fees.
