Complex sentence: James Farley was the Postmaster General under Franklin Delano Roosevelt being both were from New York they used the motto to promote the United States Postal Service during the Depression Era thus becoming its official motto.
Split into simpler sentences: James Farley was the Postmaster General under Franklin Delano Roosevelt, he was from New York and used the motto to promote the United States Postal Service. The Postmaster General used the quote to be promote the United States Postal Service during the Depression Era thus becoming its official motto.