The provisions of this Act relating to taxation of successive sales or purchases inside the State only at a single point or at one more points, shall apply only to sales or purchases inside the State (other than the sales or purchases in the course of interstate trade or commerce); and the tax under this Act shall be levied in addition to any tax levied under the Central Sales Tax Act, 1956 (Central Act 74 of 1956), or any other law for the time being in force.