Employees of the United States Postal Service, or USPS, are trained to ask customers whether the package they are shipping contains anything liquid, hazardous, perishable or breakable. This determines whether additional precautions will be necessary during the mailing process. It is legal to ship liquids via USPS, as long as you follow the proper guidelines.
Nonflammable, nonhazardous liquids are acceptable to mail, if sealed inside a waterproof container. The outer container with the address label should also be strong and securely sealed. Mark the outer container as containing liquid, so that postal staff are aware of the contents inside. Do not ship liquids sealed with only a friction-top, or push-down, seal. This type of seal is common in paint cans. Steel cans and drums are both permitted containers for liquids.
Corrosives, defined as any liquid or solid that destroys or alters human skin tissue or steel upon contact, cannot be shipped through USPS international mail. Liquid corrosives can be shipped domestically, as long as they are part of a mixture that contains 15 percent or less corrosive material. If shipped via air, the shipper must declare them on a form for dangerous goods. Flammable liquids (including cigarette lighters) are also prohibited via international and air mail, but are allowed via ground mail depending on the flashpoint of the particular liquid. Gasoline is prohibited under all circumstances.
Ship liquids in a container sealed with a screw-on top turned at least one and a half times, with a clip or by soldering. If the liquid is in a breakable container with a capacity of more than 4 fluid ounces, cushion it with material that can absorb the liquid in case the container breaks. Place the breakable container surrounded by cushioning material inside a sealed, leakproof container, and place that container within an outer mailing container.
If you want to be certain that the liquid you are shipping meets USPS regulations, use a container certified by the International Safe Transit Association, or ISTA. ISTA's Test Procedure 3A evaluates the protective performance of a packaged product in transit, according to the website of the Advance Packaging Corporation. Present an ISTA 3A Package-Product Certification Notice along with your package at any USPS office, and it should serve as acceptable verification that your liquid product has been packaged correctly.