The tax shall constitute a debt due the state and shall be due and payable nine (9) months after the decedent's death but in any case where the commissioner finds that payment of the tax within such period would impose undue hardship on the estate he may grant an extension or extensions of time for payment not to exceed six (6) months in any one (1) extension from the due date of the return or the expiration of a previous extension, if any, and all unpaid balances shall bear interest at the rate of one-half of one percent (1/2 of 1%) per month, or major fraction thereof, from the due date until paid.