Shipments that include liquids must be clearly marked as such on the outside of the package. Liquids must be sealed into a leakproof container with screw-on caps, soldering, clips or a similarly secure closure. Gasoline and any liquid classified as an explosive are entirely prohibited from being mailed through the U.S. Postal Service. Liquids that are restricted include propellants, aerosols, nail polish, anything containing alcohol, pool chemicals and paints.
Liquids that are in steel pails or drums with positive closures do not require additional packaging. Glass containers that have a capacity of more than 4 fluid ounces must be triple-packaged with a material that can absorb all possible leakage, a secondary sealed container and an outer mailing container sufficient to protect the glass from breakage, according to the USPS.
In general, liquids that are not flammable and not considered toxic, corrosive or hazardous are able to be shipped if they meet the above requirements. Intoxicating liquors do not need to be denatured prior to shipping. However, they must be shipped by Registered Mail. Perishable liquids can be shipped but may require special packaging. Some flammable liquids, such as lighter fluid, can be shipped by ground mail but not by air mail.