I learned by doing.
When I order fabric to be shipped to Canada, sometimes I have to pay a Customs fee of $5.00. I have never been charged duty, but have been charged GST (5% goods and services tax).
I never know what will happen. Most of the time the parcel comes through with no charges, but every now and again Customs will go the $5 +GST.
My understanding was because of NAFTA that we don't pay duty on things made in the USA but if they are imported to the US and then shipped to Canada we could pay duty.
If I can't get it shipped by USPS, then I don't buy because UPS fees to ship to Canada are exorbitant.
I would never mess with Customs by asking the seller to falsify. That's a serious offence.