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