Publication 52, Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 3C
A lighter equipped with an ignition element and containing fuel is classified as a Class 3 flammable liquid. A lighter containing a flammable gas is classed as a Division 2.1 flammable gas. A lighter that contains either flammable liquid or flammable gas is permitted in domestic mail via surface transportation only with prior written approval, provided all the applicable requirements of 343.25 are met.
Note: A legible photocopy of the PCSC manager‘s approval letter must accompany the mailing at the time of deposit.