I have only ever paid duty once on cards I have received from the U.S. and I have had a few orders over a $100 dollars.
The lady at the P.O. said it is a crap shoot. Some get flagged/caught and have to pay duty and others don't.
At the time, I had to pay another $28.00 on top of what I paid, which I believe was around $125 dollars worth of cards from the U.S.
I was nervous, as another set of cards were coming in a couple days for around the same amount, but, thankfully, I wasn't charged it again.
Today, I just received some more cards from PWCC that were late by 2 days so I thought for sure I was going to have to pay again, but thankfully, tonight, they were in my mail box.
I don't believe Americans pay anything more than what is listed on E-Bay whether those cards were purchased in Canada or the U.S.