Actually, all international packages sent through USPS include a tracking number. It's called a "Customs Number" but it's included even with First Class mail, all the way up to Global Express Guaranteed.
Now, actually tracking the package is something else. EMS (Priority Express) is an international standard, and therefore pretty easy to track globally. To track any item sent via USPS, you track it using the USPS website, until it leaves the US. Once that happens, the USPS will likely not update any longer, and the item must be tracked using your own country's postal tracking service.
