300 (Three Hundred S4 (Dimension 10" X 13") Poly Mailers) Tear-proof, Water-resistant and Postage-saving Lightweight Self-seal Poly Mailers/ Shipping Bags. Tough and tear-proof poly mailers. Water and tamper resistant poly mailers. Postage-saving lightweight poly mailers. High strength seams allow over-stuffing. Ink, stamps and labels adhere easily.