These Survivor庐 envelopes expand to provide lightweight, inexpensive shipping for bulky items. Expandable envelopes are made of puncture, tear and moisture-resistant DuPont鈩�Tyvek庐 material. Envelopes made with Tyvek庐 are half the weight of heavy duty paper envelopes, so you save on postage with each mailing. Our tough expandable mailers resist overstuffing, bad weather and rough handling to deliver your contents intact. With easy to use Quality Park庐 Redi-Strip庐 envelope closures, just peel and press for an instant seal. Lightweight 14 lb Tyvek庐. Seam Type: Center; Flap Type: Square; Trade Size: #13 1/2; Security Tinted: No.