Expansion Envelopes with heavyweight Tyvek offer superior protection for mailing bulky items that are not too heavy. They are ideal replacements for costly boxes or cartons. Design features an open-end style and convenient Flap-Stik closure. Envelopes with DuPont Tyvek won't rip, tear or puncture. They are much stronger and more secure than traditional paper envelopes. These 18 lb. expansion envelopes are water-resistant and moisture-resistant. They are easy to use. Simply peel and seal.