Assuming you're just mailing cards, it's almost negligible to send to the US. 96 cents as opposed to 52 cents. Large numbers of cards won't break the $1.92 mark unless you're shipping in bulk. Overseas is a bit pricier, I admit, but it's only $1.60 to send a bunch of cards. Even if you do a lot of trading, you're not going to break the bank on stamps. Also 10 days isn't going to kill anyone if they exercise a concept known as proper tournament planning.
I'm not sure on the US prices (living in Canada and all), but if they're anything like Canada's, it's stupid to say "I only send domestic." If you're unwilling to pay an extra 50ish cents, or work something out with the other person, maybe you ought to reconsider your trading.