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.
Helium is another. A balloon filled with helium will stay airborne as long as the gas inside and the equipment attached to it (basket, string, etc.) weigh less than the air in which it is floating. The balloon will rise until it reaches that height where the weight of the air outside the balloon equals the weight of the air inside it. At that point, it begins to float. Gradually, though, the air inside does begin to leak out and the balloon starts to fall to earth.