an improved method of simultaneously manufacturing a plurality of envelope assemblies of the type used to affix an insert such as a shipping label to a container . a plurality of separate strips of plastic material are arranged side - by - side on a single backing sheet of plastic material . separate layers of adhesive are affixed to portions of the backing sheet extending on either side of each strip of plastic , with a separate , removable covering strip applied to each layer of adhesive until used . each of the separate strips of plastic material is heat sealed to the backing sheet to create an envelope and slits are cut through portions of each plastic strip to provide entry into and exit from the respective envelopes . finally , the plastic strips and backing sheet are cut to separate the envelopes from one another .