If any tax or part thereof is not paid in full when due, there shall be added to the amount unpaid a penalty of five percent of the amount thereof. The entire tax or any unpaid balance thereof, together with penalty, shall immediately become collectible. Interest upon such unpaid balance and the penalty provided by this section shall be added at a rate determined in accordance with § 58.1-15, from one month after the tax was originally due until paid. In the case of an additional tax assessed by the commissioner of the revenue, if the return was made in good faith and the understatement of the amount in the return was not due to any fault of the taxpayer, there shall be no penalty on the additional tax because of such understatement, but interest shall be added to the amount of the deficiency at a rate determined in accordance with § 58.1-15, from the time the said return was required by law to be filed until paid.