First-class expansion envelopes with heavyweight Tyvek offer superior protection for mailing bulky items that are not too heavy. They are ideal replacement for costly box or carton. Design features an open-side style and convenient Flap-Stik closure. Envelopes made of DuPont Tyvek won t rip tear or puncture. They are much stronger and more secure than traditional paper envelopes. These 18 lb. envelopes are water-resistant and moisture-resistant. They are easy to use. Simply peel and seal. Quality Park Tyvek Expansion First Class Envelopes White 100 / Carton (Quantity)