If the Commission finds that an additional contribution is due, that the report was made in good faith and that the understatement of the contribution is not deliberate, no penalty shall be added because of such understatement, but the amount of the deficiency shall bear interest at the rate of one per cent for each month or fraction thereof that it remains unpaid. If the Commission finds that the understatement is due to negligence on the part of the employer, but without intent to defraud, there shall be added to the amount of the deficiency, in addition to interest, ten per cent thereof. If the Commission finds that the understatement is false or fraudulent, with intent to evade the payment of the contribution due, there shall be added to the amount of the deficiency, in addition to interest, one hundred per cent thereof.