You can't ship most hazardous, flammable or explosive liquids through USPS Priority Mail, and there are also some restrictions on perishable liquids.
Whether your business makes a beverage, perfume, soap or any other type of liquid, knowing how to ship your products legally and safely is very important. If you choose to use flat rate priority mail from the U.S. Postal Service (USPS), be certain you're aware of the restrictions on shipping liquids to ensure your packages get safely to their destinations.
The USPS will not mail items “which may kill or injure another, or injure the mails or other property," including hazardous liquids that are flammable, combustible or poisonous, but there are some exceptions to the "nonmailable" rule. Flammable liquids can be mailed in quantities of less than a gallon and if the temperature at which the liquid catches fire is between 100°F (38°C) and 141°F (60.5°C). If the liquid you wish to ship is poisonous, flammable or hazardous in any way, contact your local postal service to check if you can mail it.
General Liquid Mailing Guidelines
There must be a sticker or stamp on the outside of the package that tells the postal service that liquids are inside, even with Priority Mail Flat Rate packaging. All liquids over four ounces in glass or breakable containers must have the original container sealed within another container that won't leak, such as a plastic bag or can. Liquids mailed in glass containers must also be cushioned so they don't break. The outer mailing container must be strong enough to protect glass containers from crushing or breaking. Priority Mail Flat Rate boxes provided by the USPS are generally strong enough to contain liquids with appropriate cushioning, while Priority Mail Flat Rate envelopes aren't recommended for shipping liquids.
Special restrictions apply for items that are perishable, and foods like oil, soups or other edible liquids fall under that heading. To be mailed, the perishable items must be able to reach the destination without decaying, leaking or otherwise posting a hazard to USPS employees. Perishable goods shipped with ice need to be double packaged like liquids to prevent leakage.
Priority Mail Flat Rate
Because liquids need extra packaging to prevent leakage and cushioning for breakable containers, it's important to pick a Priority Mail Flat Rate box that's big enough to contain both the original container of liquid and the leak-proof packaging around it. Measuring 5-3/8” tall, 8-5/8” wide and 1-5/8” deep, small Priority Mail Flat Rate boxes might be suitable for small quantities of liquid such as perfumes or herbal tinctures. For larger bottles of liquids like wine or soda, a large Priority Mail Flat Rate box might be more suitable.