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 helium that is available commercially is extracted from oil and gas wells. Helium can be extracted from underground because it is trapped by the same sorts of impermeable rock layers that trap petrochemicals. That helium has had thousands or millions of years to accumulate in those rock formations, but we can empty them in a few decades.