United States Postal Service (USPS)
Only the USPS accepts cremated human remains for shipment. They must be sent Priority Mail Express and cannot be sent overnight express, regular, or certified mail.
The cremated remains must be packaged in a sift-proof or similar-type container. The container must be sealed inside a second, durable outer container and a copy of the cremation authorization form must be enclosed. Many crematories have containers specifically designed to ship human remains.
Transporting on Airlines
Because of required Transportation Safety Agency (TSA) screening procedures, airlines treat cremated human remains as carry on luggage only. Cremated remains may not be checked through as regular luggage.
The cremated remains should be transported in a temporary plastic or cardboard container that will pass the X-ray screening process. Standard urns and opaque containers typically fail the screening process. You may also need to remove the remains from your carry-on luggage, otherwise you may be asked to remove them by a TSA agent.
Remember to check with the airline for any additional requirements.