As someone who once worked with FedEx, the high cost of sending anything international goes primarily to customs. To send a simple letter within the US costs around $25 for overnight. For the same letter to go to Canada, $48. For it to go to the UK, it costs $90. That's also the same for it to go to China, Japan, El Salvador, Germany, Sweden, Russia, etc. The cost breakdown is about 35% to customs (that's $30), 35% to actual cost (fuel, equipment, etc), and about 15% to salaries.