These expansion envelopes are ideal for mailing bulky items such as binders, apparel, parts and more. These light-weight envelopes provide an excellent alternative to heavy boxes. Square flaps use self-adhesive to fully close and secure inner contents. Deluxe material resists moisture, punctures, tearing, chemicals and burns.These expansion envelopes are ideal for mailing bulky items such as binders, apparel, parts and more. These light-weight envelopes provide an excellent alternative to heavy boxes. Square flaps use self-adhesive to fully close and secure inner contents. Deluxe material resists moisture, punctures, tearing, chemicals and burns.