Approved by U.S. Postal Service, these Expandable Transit Sacs are ideal for larger materials like bulk mail, samples, small parcels, or office supplies. Sacs can be sent through the mail or used by courier/messenger services. Constructed of Laminated Nylon, a 3-ply, 70-denier fabric with waterproof polyvinyl coating and cotton twill backing, these sacs will stand up to years of rough handling. Sturdy zipper closure keeps contents intact. Keep contents secure and confidential by looping a Plastic Security Seal through grommet and zipper pull. Clear, framed acrylic I.D. window to indicate routing information.