There are some rules and regulations that need to be followed when shipping liquids via the United States Postal Service. The USPS has the following guidelines to follow when shipping liquids:
a.) Flammable Liquid means a liquid that has a flashpoint of not more than 141° F (60.5° C), or any material in a liquid phase that has a flashpoint at or above 100° F (38° C).
b.) Combustible Liquid means any liquid that does not meet the definition of any other hazard class and has a flashpoint above 141° F (60.5° C) and below 200° F (93° C). A flammable liquid with a flashpoint at or above 100° F (38° C) that does not meet the definition of any other hazard class may be reclassified as a combustible liquid per 49 CFR 173.120(b).
Note: A liquid with a flashpoint above 200° F (93° C) is not regulated as a hazardous material and may be mailed subject to the general packaging requirements in DMM 601.1–8, provided it possesses no characteristics of another hazard class.
International Mail: Any Flamable or Combustible Liquid is prohibited from being shipped via International USPS mail. Any liquid that combust below 200°F cannot be shipped. Combustible Liquids may be shipped using UPS Internationally, where they are not considered hazardous.
Domestic Mail: To ship Combustible Liquids with the USPS you must use Parcel Post, and the package must be clearly labeled with the correct flashpoint identified on the outside of the package. In addition, the package must have ORM-D Parcel Post only clearly labeled on the outside of the carton.
Violations of these regulations could be considered fraud, and the sender could risk paying fines of up to $10,000 or even imprisonment. Thus, it is important to know the postal laws before you decide to ship any type of combustible liquid.