A while back I imported some nice perfumes for the mrs. through a website that didn't pay for duties and such in advance. Total price incl. shipping was about 18.000 baht, and after the customs got a hold of it, I had to pay an additional 10.000 baht in duties and VAT in ordet to receive the package.
So Amazon's 50% probably isn't unreasonable, depending on what products you're trying to bring in.