“Its” refers to the United States Federal Government and is possessive
Updegrave 91 (W.C., “Explanation of ZIP Code Address Purpose”, 8-19, http://www.supremelaw.org/ref/zipcode/updegrav.htm)
More specifically, looking at the map on page 11 of the National ZIP Code Directory, e.g. at a local post office, one will see that the first digit of a ZIP Code defines an area that includes more than one State. The first sentence of the explanatory paragraph begins: "A ZIP Code is a numerical code that identifies areas within the United States and its territories for purposes of ..." [cf. 26 CFR 1.1-1(c)]. Note the singular possessive pronoun "its", not "their", therefore carrying the implication that it relates to the "United States" as a corporation domiciled in the District of Columbia (in the singular sense), not in the sense of being the 50 States of the Union (in the plural sense). The map shows all the States of the Union, but it also shows D.C., Puerto Rico and the Virgin Islands, making the explanatory statement literally correct.