Can mail liquid through USPS?

Mailers must mark the outer container of a mailpiece containing liquid to indicate the nature of the contents. Mailers must package and mail liquids under the following conditions:

a. Use screw-on caps, soldering, clips, or similar means to close mailpieces containing liquids. Do not use only friction-top closures (push-down types).

b. Liquids in steel pails and drums with positive closures, such as locking rings or recessed spouts under screw-cap closures, may be mailed without additional packaging.

c. Package glass and other breakable containers of liquid with a capacity of more than 4 fluid ounces according to the following requirements:

1. Cushion the primary container with material sufficient to absorb all leakage in case of breakage.

2. Place the primary container inside another sealed, leakproof container (secondary container), such as a can or plastic bag.

3. Use an outer mailing container that is strong enough to protect the contents.

d. As an alternative to 2.4c above, mailers may use containers certified by the International Safe Transit Association (ISTA) that passed ISTA's Test Procedure 3A. Mailers must provide their ISTA 3A Package-Product Certification Notice at the time of mailing as verification that the mailpieces they are submitting passed the required performance test.

There are other topics that list restrictions or prohibitions for some types of liquids. You did not mention what kind of liquid you are trying to send, nor did you say where it would be shipped. This info could make a huge difference in my answer.

Mailing Standards are available online in our Domestic Mail Manual or International Mail Manual.

Source: https://au.answers.yahoo.com/question/index?qid=20080520164752AAexXHb