If you use the paypal method (https://www.paypal.com/?cmd=_ship-now), select USPS, First Class international, package size Large Envelope, and I usually select a generic 2-3 oz, throw it in a bubble mailer, and it comes out to $2.82 with shipping.
Edit: Don't forget to to sign it on the customs line, and I usually put the value at $1, and call it a "photo" or something, so that they don't get any additional charges from customs (and no douchey postal workers get any ideas.)