The speed of mail traveling between Guam and the states varies depending on size and time of year. Light, first-class items generally take less than a week to or from the mainland, but larger first-class or Priority items can take a week or two. Fourth-class mail, such as magazines, are transported by sea after reaching Hawaii. Most residents use post office boxes or private mail boxes, although residential delivery is becoming increasingly available. Incoming mail not from the Americas should be addressed to "Guam" instead of "USA" to avoid being routed the long way through the U.S. mainland and possibly charged a higher rate (especially from Asia).The Commercial Port of Guam is the island's lifeline because most products must be shipped into Guam for consumers. The port is also the regional transhipment hub for over 500,000 customers throughout the Micronesian region. The port is the shipping and receiving point for containers designated for the island's US Department of Defense installations, Andersen Air Force Base and Commander, Naval Forces Marianas and eventually the Third Marine Expeditionary Force. Guam is served by the Antonio B. Won Pat International Airport, which is a regional hub for Continental Micronesia. The island is outside the United States customs zone so Guam is responsible for establishing and operating its own customs and quarantine agency and jurisdiction. Therefore, the U.S. Customs and Border Protection only carries immigration (but not customs) functions. Since Guam is under federal immigration jurisdiction, passengers arriving directly from the States skip immigration and directly proceed to Guam Customs and Quarantine. However, due to the Guam and CNMI visa waiver program for certain countries, an eligibility pre-clearance check is carried on Guam for flights to the States. For travel to and from the Northern Mariana Islands (which have separate immigration regulations), a full inspection is performed though American citizens do not need a passport. Traveling between Guam and the States through a foreign point (for example, a Japanese airport), however, does requires a passport. Most residents travel within Guam using personally owned vehicles. The local government currently outsources the only public bus system (Guam Mass Transit Authority), and some commercial companies operate buses between tourist-frequented locations.