Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 2A
A Class 2, Division 2.1 flammable gas that qualifies as a Limited Quantity surface material is mailable provided that all applicable requirements in 342 are met and it is properly packaged as follows.
Labels and markings must be placed on the address side of the mailpiece unless specified differently in 221.1 and 325.1.