Our strongest mailing bags are comprised of 90 (354 gauge) micron polythene, a much tougher and more durable material than that of standard mailing bags. All of these heavy duty bags are co-extruded, which means that several layers of film are extruded together to create a more robust bag. Kite offer six sizes, all of which comply with Royal Mail PiP guidelines, enabling you to accurately calculate your mailing costs.