I estimated the weight of the book by using a book that is the same size and nearly the same page count. Similar methods were used to estimate the weight of other rewards.
International shipping costs are brutal, and for that I apologize. I wish they were less, but that's the way the cookie crumbles. I did not add any extra cost beyond a dollar or less for shipping supplies and, for international orders, a couple bucks for insurance.
In some instances, I will be shipping the book but other parts of the reward (like custom artwork) will be shipped by the artist. In these cases, shipping may be slightly higher to accommodate two separate packages. Whenever possible, I will collect rewards from local artists and ship them in one package to save on shipping (which is reflected in the shipping cost of the reward).