N.Y. Veh. & Traf. Law § 1229

§ 1229. Sign required on privately-owned vehicle used in delivery of\nmail. 1. Every person operating a privately-owned vehicle in delivering\nthe United States mail to the addressee thereof shall affix to such\nvehicle a white sign, visible from the rear, containing the following\nstatement:\n U. S. MAIL\nSuch statement shall be in black lettering at least four inches in\nheight.\n 2. Every person who violates the provisions of this section shall be\npunishable by a fine not exceeding twenty-five dollars or by\nimprisonment for not more than ten days or by both such fine and\nimprisonment.\n