Shipping sucks no matter how you slice it. Rob was correct when he said the problem will NEVER be solved until KS changes the method but unfortunately they wont change it.
I have shipped 1 or 2 decks and my advise would be to charge a fair and reasonable price for your shipping. Even if you provide free shipping there will 300 people who gripe that it is too expensive. If you think I'm kidding or being sarcastic, I am sadly not. Do over charge for it and DO NOT under charge as that will lead to you going bankrupt and then no one will get any rewards and that is worse than expensive shipping.
This problem is a real shame, and even more so because international backers are just used to it at this point as Rose said. I know that personally I wanted to back for many more decks of a few projects because I knew I would use and love using them, but it can spiral in relation to cost.
In your experience, were fulfillment companies something you considered? I know I could easily partner with a KS project to fulfill projects in Europe for about $5 for 1-4 decks and $10 for larger, this also meaning that no VAT would need to be paid by a backer in Europe on their 'purchase' by EU law, but it doesn't seem like KS are built for allowing this kind of thing to occur, ie different rates for EU customers.
PS. I wasn't asking this with the idea of me setting up a playing card project in mind (art skills so not good enough for that!) but I hope maybe someone who is may see your post and be saved some troubles!