USPS will be the cheapest by far, but they can only track and insure the goods while they are inside the USA so if they get lost after that then it's your problem.
Fedex and UPS will ship overseas and give you online tracking right up to the person's front door, but you will pay a hell of a lot more (up to 3 times as much!). I sent a laptop to my brother in England with insurance and tracking and it came to $190 (discounted as they were having a "special" that day). USPS was $60.
Personally, I wouldn't ship to someone off eBay without tracking and insurance or you're asking for trouble.