A package of flammable liquid materials will have a Class 3 flammable liquid placard visible on the outside of the package. The placard is an official image that is a red diamond with a white flame on top and the number three on the bottom. The placard may say "flammable" between the flame and the number.
A Class 3 flammable liquid is any liquid that has a flash point of 141 degrees Fahrenheit or below. The flash point of a liquid is the minimum temperature at which the liquid will give off vapor. This means that the liquid will easily become inflamed when exposed to high temperatures, to an open flame, or when it is put under a significant amount of pressure. Liquids that are of Class 3 include fuels, many paints, hairspray, solvents and alcohols. Liquids that have a flash above 141 degrees but below 200 degrees are considered "combustible" liquids.
According to the United States Postal Service, it is legal to mail Hazard Class 3 flammable liquids domestically as long as they are packaged and labeled correctly. Flammable liquids can only be mailed by ground services and not by air, and international mailing of these liquids is prohibited. Some Class 3 flammable liquids may be eligible to be reclassified as combustible liquids if they are less likely to become enflamed.