There's more then the actual shipping cost involved in shipping. If it costs $17 CDN from the post office to ship I am charging $20-$22 CDN. Bubble mailer, top loader, sleeve, team bag, my time to package, gas, wait time at the post office, etc. It's all a factor that is passed onto the buyer. Don't like it, as the other fellow said, don't buy from me.
I'd also like to say that I too sell cards, primarily on eBay. In fact, I'm sending a Lundqvist RPA out to Red Deer, Alberta today. That being said, I never charge more than I actually pay for shipping. And eBay rules don't allow for charging for gas and wait time at the post office.
This argument has been going on for time immemorial... bubble mailers and supplies cost mere cents per unit, I give my packages to my mailman, or the post office is cents of gas away, and I never have to wait, as I print at home using PayPal.
A tracked package to the US from Canada, even if you figure in the most liberal estimates for time and supplies would not exceed $18.26 CAN. If you're spending over a dollar per shipment for supplies and even time and travel, you're doing it wrong.
Example of how it should be done: a couple of years ago, UD had a dual Dionne/Esposito Signature Renditions card /15. I probably paid twice what it should have gone for, but I wanted it, and only saw one or two others ever come up for sale since. But the seller was asking an exorbitant amount for shipping. Got in touch, explained to him the situation, pointed out my flawless feedback, and after a reasonable conversation, he split the shipping costs with me, even sent it untracked at my request, and it arrived safely, and he made a solid hefty profit from it, and all was right with the world.
If you don't want to ship to the US and want to discourage Americans via high shipping rates, fine. If you want to profit off of shipping, even if you're not allowed to, fine. Just be honest and say so, and let's move on. But stop with the defensive and rude attitudes about it, you're only costing yourselves money.