a method for forming a tamper - resistant envelope formed of a thermoplastic resin comprises two opposing panels having respective bottom and side wall edges bonded together to form a pocket therebetween . the top of the pocket is open for inserting an item into the pocket . each panel has an edge portion adjacent the opening . a bead of pressure - sensitive adhesive is applied adjacent the edge portions , and the bead is heat - seal bonded to the thermoplastic edge portion . the edge portions can be brought together to bring the adhesive beads into contact to close the opening . by heat - seal bonding the adhesive beads to the thermoplastic edge portions , the pressure - sensitive adhesive is less susceptible to being opened by supercooling .