The TSA rules are that when transporting cremated remains they must be in a container that allows the contents to be viewed by their scanners (no metal). They must be hand carried (never packed in your luggage). It is true that they are only cremated remains but from the TSA's point of view they look like something that might go boom. Using the carboard container that the crematory provides along with the necessary papers (certificate of cremation and state permit) will suffice for domestic traveling. If you have the cremated remains in an urn already forwarding them via USPS to your destination is a plus. (FedEx and UPS will not accept cremated remains for shipment).

For overseas or foreign transportation you can either contact the embassy and get the necessary forms and permits from them (as well as their subsequent fees) or send them to the location via USPS and thereby circumvent the process.