I think you're confusing "terminal dues" (which is what countries charge each other for delivering a package in the destination country) and the actual cost of shipping between countries (which is what I'm talking about here)

Since you are responding to someone comparing US domestic (in town) shipping prices to international shipping price, I don't see how you are not talking about "terminal dues".

International freight costs could explain why international shipping is more expensive than domestic shipping, but even if international freight prices are 0, they can't explain why domestic shipping is more expensive than international shipping.