Re: USA traders, why do you count out Canada?
There are many reasons that international shipping is more of a hassle. Even just going to Canada packages have to pass through customs which can cause delays. Shipping prices seem to increase significantly if you want any sort of tracking or insurance. First Class International can get it there cheaply, but you run the risks involved with sending an item without tracking.
I used to sell jewelry, if you put the value of the item on the customs form and you don't have tracking or insurance, you're just inviting people along the way to mess with it. In addition to that if you do ship with a customs form anything over $25 to Canada and don't mark it as a gift, it's possible that the person receiving the item will have to pay import duties or taxes. This seems to be inconsistent as I once shipped a $200 bracelet and had no complaints, but then $30 worth of earrings had the person e-mailing me that she paid as much in fees as she did for the earrings. Both items went to Canada, both were shipped from the same post office and both had customs forms. I had a $100 item go 'missing' that was shipped to another country, and was basically just told sorry and that they couldn't track it since I didn't buy the $15 insurance.
More recently I paid $8 to ship a video game to Canada and it returned to me because the buyer didn't put his apartment number in the order. So in order to provide good customer service and keep positive feedback, I paid for the second shipping out of pocket ($16 total to ship a $40 item. I had only charged $7 shipping). So sending cards for cards and paying fees/extra shipping on top of it just seems to negate the point of trading. I could just be buying cards and adding to my collection instead. I have sent small trades to Canada though. If it's less than 6 cards and I can feel comfortable sticking it in a plain white envelope, then it's only a little over $1 or three regular stamps.
One thing I have learned to do though is order my shipping labels through PayPal's Multi-Order shipping. PayPal gets you a commercial rate on your shipping which saves money and includes some tracking information even on cheaper packages. It also avoids the argument I seem to have with some postal workers as to what counts as a package, since some of the bubble envelopes aren't quite 1/4 of an inch thick.