Regular-weight Expansion Envelopes offer superior protection for mailing bulky items which are not too heavy. Ideal replacement for a costly box or carton. Convenient Flap-Stik closure requires no moisture to seal. Simply peel and seal. Much stronger and more secure than traditional paper envelopes, these first-class envelopes are made of DuPont Tyvek and won't rip, tear or puncture. Expansion envelopes are also water-resistant and moisture-resistant. Open-end style offers easy insertion.