Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 2B
A Class 2, Division 2.2 nonflammable gas that qualifies as a Limited Quantity air or Limited Quantity surface material is mailable provided that all applicable requirements in 342 are met and it is properly packaged as follows.
The following labels and text markings must be placed on the address side of the mailpiece unless specified in 221.1 and 325.1.