I recently purchased a large collection containing many mint stamps. Some of the stamps were from the late 1990s and early 2000s. The stamps were still in their USPS packaging – mostly the blue and white cardboard backing with cellophane wrapping.
I removed all of the material from the packaging. In doing so, however, I noticed that a few of the items were lightly stuck to the cellophane. This is because some of the panes of stamps were packed tightly together in a box. Luckily, I was able to remove them without damage.
Today, the USPS doesn’t wrap the stamps that are sent to post offices. If you want the stamps, you ask the window clerk and they pull what you want from their loose stock. However, if you order stamps from the Stamp Fulfillment Center in Kansas City, MO those items are still shipped with cardboard backing and a cellophane wrap.
Neither the cardboard backing nor cellophane wrap are meant for long term storage of stamps. Being wrapped in cellophane makes the stamps sweat even more if they are in a humid environment. Some of the stamps from the 1990s and 2000s were water activated gum, meaning they have even more potential to stick to their packaging.
If you have material that is still wrapped in the USPS packaging, I encourage you to remove it and properly store those items. Leaving material in the USPS packaging for an extended period of time is just asking for trouble. It may ruin your stamps.