A couple reasons for the extra postage. One, they can get away with it. Most people don't object to paying more when the item is going to an international destination. Most politicians will get every penny they can out of the people if they don't object. Secondly, it does cost more as most countries have some sort of security on incoming international mail which means paying for employees to make sure letter bombs, etc. aren't
coming into the country. Also they pay the airlines to deliver the mail, at least in countries like ours where the airlines are not government-owned.