My experience with various USPS options as someone living in Europe, although it most likely varies from country to country (sorry if it's too OT):
1. First Class Mail International - about 1-2 weeks delivery, can be tracked but only while its still in US, once it goes through the sorting facility out of US there will be no more updates
2. Priority Mail International - delivery time ranges between couple days and a week or so, has different kind of tracking number than First Class and if your local postal service supports it (mine does), can be tracked all the way to the destination
3. Express Mail International - takes only few days, the only sure way to get proper tracking all the way to the destination, but too expensive to be worth it for items of lesser value
Customs however are quite a gamble and can sometimes hold up a package for ages, even if it only took few days to get to your country.