(a) If any person believed by the department to be liable for tax under the provisions of this act shall have failed to file a return in accordance with and within the time prescribed by this act, and if the department shall deem it more conducive to the public interest because of the supposed smallness of the tax or for any other reason not to proceed to compel the exhibition of the accounts of such person, it may make an estimated assessment of the probable amount of tax owing by such person; but in every such case the department shall add to such estimated assessment a penalty of fifty per centum (50%) thereof and the department shall proceed to collect such estimated tax and penalty as in other cases if the amount is not paid when due.

(b) The estimated assessment thus determined, together with the penalty of fifty per centum (50%) specified above and interest at the rate of one-half of one per centum per month or fractional part thereof until paid, shall be due and payable ten days after notice of such settlement shall have been mailed by the department to the person against whom the estimated assessment has been made.