Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 6A
A Division 6.1 toxic substance that qualifies as a Limited Quantity material within the Postal Service is permitted in the domestic mail via air or surface transportation when all applicable requirements in 346 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.