OEM Quality Park Heavyweight Expansion Envelopes provide superior protection for mailing bulky items. Made with heavyweight Tyvek, these envelopes are designed to be much stronger and more secure than traditional paper envelopes. They feature an open-side style and a convenient Flap-Stik closure, ensuring easy use with a simple peel and seal mechanism. Water-resistant and moisture-resistant, these envelopes are ideal for replacing costly boxes or cartons.