Approved by U.S. Postal Service, these Flat Transit Sacs are ideal for flat materials like letters, reports, memos, publications or important documents. Sacs can be sent through the mail or used by courier/messenger services. Sturdy zipper closure keeps contents intact. Keep contents confidential by looping a Plastic Security Seal through grommet and zipper pull. Clear, framed acrylic I.D. window to indicate routing information.