Expandable mailer made with DuPont® Tyvek™ material delivers the best possible protection at a shipping-cost-saving weight. Inserting and removing contents is easy because of its horizontal, open side orientation. Self-adhesive Redi-Strip™ closure provides a secure, peel-and-press seal. Clean, crisp look makes a professional impression. USPS First Class Mail—Heavyweight 18 lb Tyvek™. Seam Type: Center; Flap Type: Square; Trade Size: #13 1/2; Security Tinted: No.