First-class 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-end style and convenient Flap-Stik closure. Being much stronger and more secure than traditional paper envelopes, these envelopes with DuPont Tyvek won't rip, tear or puncture. First-class expansion envelopes are water-resistant and moisture-resistant. They are easy to use. Simply peel and seal.