Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 9C
A Class 9 miscellaneous hazardous material that qualifies as a Limited Quantity material within the Postal Service is permitted in domestic mail provided all applicable requirements in 349 are met.
The following labels and text markings must be placed on the address side of the mailpiece unless specified in 221.1 and 325.1.