On or before the tenth day following each month in which such advance was furnished to the officer or employee, he or she shall submit to the head of his or her department a fully itemized travel expense voucher fully justifying the expenditure of such advance or whatever part thereof has been expended, for legally reimbursable items on behalf of the state. Any unexpended portion of such advance shall be returned to the agency at the close of the authorized travel period. Payment shall accompany such itemized voucher at the close of the travel period; and may be made by check or similar instrument payable to the department. Any default in accounting for or repaying an advance shall render the full amount which is unpaid immediately due and payable with interest at the rate of ten percent per annum from the date of default until paid.