The time limit for. shipping a POV is the same as for travel and HHG transportation limits (JTR Para 5366, POV. Transportation Time Limitation), 1 year from the date of retirement/separation with severance. pay or 180 days from the date of separation.
The envelope is the large bag that holds the helium gas. The envelope is generally cigar-shaped, for aerodynamic purposes, and made of a durable, airtight, lightweight fabric (polyester composites) that is much like the fabric of a space suit.
The Earth is constantly degassing, which transports helium from the crust and mantle into the oceans and atmosphere. Because it is such a light atom, helium escapes from Earth and is thereby continually lost from the atmosphere. The approximate lifetime of helium in the atmosphere is ~ 1 to 2 Myr.