I believe it has something to do with the volume of mail originating from a country, and some sort of international agreement between countries.

A country like Canada with 10% (?) the population of the US honours the US postage of letters addressed to Canada and delivers them for 'free'. Because the US has such a large population in comparison, they get a 'volume' discount for international letters. Canada, with it's smaller population would have to pay more for each letter.

It's been a long time since I've worked at the wicket (selling stamps and sending parcels) so I can't say for sure...

Anyways, there's a price for living in Paradise isn't there