who has the power to establish post offices
Postal Clause Article I, Section 8, Clause 7 of the United States Constitution, known as the Postal Clause or the Postal Power, empowers Congress "To establish Post Offices and post Roads".[1]
Old Post Office (Washington, D.C.) The Old Post Office, listed on the National Register of Historic Places as the Old Post Office and Clock Tower and located at 1100 Pennsylvania Avenue NW in Washington, D.C., was begun in 1892, completed in 1899, and is a contributing property to the Pennsylvania Avenue National Historic Site.[1] It was used as the city's main General Post Office until 1914 at the beginning of World War I, succeeding an earlier 1839 edifice, G. P.O. of Classical Revival style, expanded in 1866 on F Street, which later was turned over to the Tariff Commission and several other agencies (today, the Hotel Monaco). The Pennsylvania Avenue 1899 landmark structure functioned primarily as an federal office building afterward, and was nearly torn down during the construction of the surrounding Federal Triangle complex in the 1920s. It was again threatened and nearly demolished in the 1970s to make way for proposals for the completion of the enveloping Federal Triangle complex of similar Beaux Arts styled architecture government offices, first begun in the 1920s and 30s.