4 1/2 x 6″ Red “Important Invoice and/or Packing List Enclosed” Envelopes. Pressure sensitive packing list envelopes secure and protect documents that are attached to the outside of shipments. Use these 2 Mil poly envelopes to attach packing lists and/or invoices to the outside of packages. Hot melt adhesive backing provides strong adhesion to paper and corrugated products. Envelopes open along the first dimension.