sending cremated remains to the U.S.

Recently a friend of mine died here in Spain. He had requested cremation in his living will, but didn't specify what to do with his ashes. Upon informing his U.S. family, they requested that his ashes be sent to them. I've done some internet research and found that the U.S. postal service does ship and accept cremated remains. But I haven't been able to find if Correos accepts them for shipment (UPS does not.)

Any advice would be appreciated.