It does not cost $20 to post two cards across the Atlantic - I know, I have posted individual cards over several times (and received such many more times). The $20 for charity I have no issue with - bloated shipping costs, I do.
For those of you with experience shipping cheaply across the ocean, maybe think about dropping an email to the store? The people who run it (Rothfuss and his assistants) run the store entirely for charity - I doubt that they are looking to make fat cash off bloated shipping. Maybe if they knew a better way to go about it, they'd take it.