Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 3A
A flammable liquid having a flashpoint greater than 20° F (–7° C) but less than 100° F (38° C) is mailable in domestic mail via surface transportation, if the liquid qualifies as a Limited Quantity surface material, and all applicable requirements in 343 are met. Flammable liquids having a flashpoint of 20° or less are not mailable.
Separate packaging requirements apply depending on the flashpoint of the flammable liquid.
Labels and markings must be placed on the address side of the mailpiece unless specified differently in 221.1 and 325.1.