Expansion Envelopes with heavyweight Tyvek offer superior protection for mailing bulky items which are not too heavy. Ideal replacement for costly box or carton. Envelopes feature open-side style and convenient Flap-Stik closure. Envelopes made with DuPont Tyvek won’t rip, tear or puncture. Much stronger and more secure than traditional paper envelopes. Water-resistant and moisture-resistant. Easy to use, just peel and seal.