When the package is ready to ship, the vacuum is released, allowing shipper to expand and create a custom fitted package. Said to provide protection against shock and vibration during shipment as well as extreme temperatures.
Xpander Pak: Insulated shipper
The Xpander Pak, RX version, from Xpander Pak (Richmond, VA) has a durable film outer wall and a flexible inner wall. Enclosed in the space between the sides of the package is an open-celled, fully flexible foam held in a compressed condition with a vacuum.