Publication 52, Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instructions 10C
Human and animal ashes are permitted for mailing with restrictions, provided they are properly double-packaged and labeled. For international shipping, cremated remains are permitted to be mailed provided they are not otherwise prohibited by the destination country (see the Individual Country Listings in the IMM) and all packaging and marking requirements are met.
Note: A siftproof container is any vessel that does not allow loose powder to leak or sift out during transit.
Note: It is recommended that all packages contain a slip of paper with the sender’s and recipient’s address and phone number. This will provide Postal employees with a means to contact you if the outer mailing label is damaged or found missing during transit.