Each of the major carriers handle international orders differently. For instance, USPS partners with other countries’ public postal services to carry out final delivery. They are able to do this and offer some of the lowest international rates because of the United States’ membership in the Universal Postal Union (UPU).
Conversely, other carriers like UPS, FedEx, and DHL rely on their own networks to carry out delivery to the final international destination, instead of working with foreign countries’ federal postal services. Since these larger carriers utilize their own logistics networks, they are able to offer faster, more trackable, and more reliable international deliveries (but they are almost always more expensive).