Provided that, if the registration certificate holder fails to pay the required registration certificate retention fee on or before due date, the registration certificate holder shall, in addition to the registration certificate retention fee, be liable to pay a late fee calculated at the rate of two per cent. of the registration certificate retention fee for every month or part thereof within six months: Provided further that in the event of non-payment of such fee within the period referred to in the first proviso, the registration certificate shall be deemed to have been cancelled.