Jan van Leyden
Adventurer
The best idea so far seems to handle it like Evil Hat is doing for Designers & Dragons: shipping will be charged separately after the Kickstarter using Backerkit or any other website for fulfillment of kickstarters.
That way shipping doesn't screw your numbers during the campaign and you can have international orders. You just have to be very careful when calculating shipping costs so backers know what to expect more or less.
This way you only shift the liability to the supporter. "OK, I buy this book for $40, but don't have the slightest idea how expensive it will finally be."
Add the fact that a lot of Kickstarter projects are way behind schedule. Say, I support project for $40 + shipping to Europe. I research the possible cost and calculate $20 for shipping; the thing would cost me $60. But what if the promised delivery date shifts from November 2014 to November 2015 which increases the shipping cost from $20 to $30?
For me personally this just means that besides not knowing what I will get and when, I either don't know how much it will cost me. Not very attractive...