Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 2E
An audible fire alarm system powered by a compressed gas is mailable only in domestic mail via surface transportation provided that the system qualifies as a Limited Quantity surface material and all applicable requirements in 342 are met.
Labels and markings must be placed on the address side of the mailpiece unless specified differently in 221.1 and 325.1.