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. Envelopes feature open-side style and convenient Flap-Stik closure. Envelopes made with DuPont Tyvek won t rip tear or puncture. They are much stronger and more secure than traditional paper envelopes. These 18 lb. envelopes are water-resistant and moisture-resistant. They are easy to use. Simply peel and seal.