Every spool of Fillamentum is shipped in a re-sealable vacuum bag made from BoPET (Mylar). This is the most suitable material for packaging the spool because it prevents UV light from degrading the filament and also serves as a good gas barrier to minimize moisture from getting into the bag.
The best way to open the bag is to use a sharp pair of scissors to cut it open. Avoid tearing it open as the violent action could cause damage to the seal.