Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 10C
Human and animal cremated remains in any state (e.g. ashes, keepsakes, and jewelry) are permitted for mailing with restrictions, provided they are appropriately prepared according to 451 and the following instructions.
Not applicable.
Note: A sift-proof container is any vessel that does not allow loose powder to leak or sift out during transit.
Note: It is recommended that you attach a slip of paper to the sealed plastic bag with the complete return and delivery addresses and the words “Cremated Remains” in the event the mailing label becomes detached from the outer packaging after acceptance.
Labels and markings must be placed on the address side of the mailpiece unless specified differently in 221.1 and 325.1.