20 Grey Mailing Bags 12x16 Strong Quality Strong UK manufactured mailing postal bags - perfect for sending items in the post or via courier items placed in the bag are not visible through the material Durable and Water Proof: Our grey postage bags are a wonderful choice owing to their high-strength design. Our mailing bags feature 3 layers of polythene film into one making them very robust, opaque, and durable. These are designed to keep the contents secure against moisture, chemicals, tampering, puncturing, and tearing as these are made strong enough to withstand the rigours of the shipping process.