OEM Quality Park Heavyweight Expansion Envelopes provide superior protection for mailing bulky items that are not too heavy. Made with heavyweight Tyvek, these envelopes are much stronger and more secure than traditional paper envelopes, ensuring they won't rip, tear, or puncture. The open-side style and convenient Flap-Stik closure make them easy to use—just peel and seal. Additionally, they are water-resistant and moisture-resistant, making them ideal for various mailing needs.