I believe the post office (in the US, at least) charges based on weight, what little additions you want (insurance, tracking, etc), and priority. It's probably not possible for you, but my dad has a scale and does his postage online with Stamps. Sadly a handy thing we don't have our disposal. :\

And it's gonna cost more to send overseas (but that was obvious.). Maybe charge a tiny bit extra for stateside postage so we can lower costs for the members in different countries?