Block’s Transit Sacs are a reusable solution designed to survive hundreds of round trips – the sacs can hold large capacities and enable you to color code mail. Transit sacs are approved by the U.S. Postal Service and can save you 96% annually on envelope costs and 37% on postage costs. These bags feature zipper closures that keep contents secure while the combination locks or security seals guarantee confidentiality. The bags come standard with an I.D. window and address card.