I ship maybe a dozen or so packages overseas in a month. USPS is usually the cheapest and the least hassle. However, people in some countries will request that you use other carriers depending on how much luck they've had with their local post office.
I've had to ship internationally by FedEx occasionally and even if you go over all the paperwork with their employees to make extra sure it's all filled out correctly, someone along the way may decide that you didn't follow all of their rules exactly and send it back. Then, not only are you out the shipping expense, the person who was in a hurry to get your package probably won't get it on time.