The World’s Largest Post Office Will Fit In The Palm Of Your Hand Now. By Downloading The New U.S. Postal Service’s Mobile App Onto Your Smartphone. After Installing The New App,Informed Delivery, At the App Store, You Can Buy Stamps, Calculate Postage, Ship Packages And Track Packages. Postal Officials Estimate That From Now Through New Year’s Day, More Than 15 Billion Pieces Of Mail Will Be Delivered.