The tax levied by RCW
82.08.020 shall not apply to sales to corporations which have been incorporated under any act of the congress of the United States and whose principal purposes are to furnish volunteer aid to members of armed forces of the United States and also to carry on a system of national and international relief and to apply the same in mitigating the sufferings caused by pestilence, famine, fire, floods, and other national calamities and to devise and carry on measures for preventing the same.