The tax payable under section 5 and section 5A shall be increased by an additional sales tax at the rate of fifteen per cent of the tax payable under the said sections:

Provided that no additional sales tax under this section shall be levied on the tax payable on High Speed Diesel Oil, Petrol falling under sub-entries (ii) and (iv) of Sl. No.1 and Foreign Liquor falling under Sl. No.2 of the Schedule.