Dry ice is often used to keep items cold during shipping. The United States Postal Service (USPS) allows you to ship dry ice domestically, within certain restrictions. Dry ice is considered a hazardous material and is subject to the USPS regulations concerning shipping hazardous materials.
The USPS does not allow you to ship dry ice internationally. Alternatives to dry ice include self-contained gel coolants, like those often used to keep children's lunch boxes cool. Always check for compliance with the USPS before using any alternate coolant.
Domestic Air Mail
Any package with dry ice that travels by air must contain five pounds of dry ice or less and must be appropriately packaged. Proper packaging involves using enough insulation to ensure that the mailing carton does not become wet from condensation. Additionally, the container must be made of a material such as fiberboard that allows the release of carbon dioxide gas. The package must also be clearly marked with a Department of Transportation warning label that says either "Carbon Dioxide Solid, UN1845" or "Dry Ice, UN1845." Finally, you must complete the appropriate triplicate shipper's form, available at the USPS, declaring that you are shipping dangerous materials and attach a copy to the outside of the shipping container.
Domestic Surface Mail
Pieces of mail with dry ice that are specifically marked "Surface Mail" or "Surface Mail Only" may contain more than five pounds of dry ice. They must be marked "Carbon Dioxide Solid, UN1845" or "Dry Ice, UN1845" like their air mail counterparts and must be similarly packaged to prevent condensation and moistening. No shipping declaration or DOT Class 9 warning is needed for surface shipping.