Assessment of non-filers: tax may be determined to best judgment; filing later doesn't remove interest or late fee. If a registered person fails to furnish the return after notice, the proper officer may assess the tax liability to the best of his judgement using relevant material and issue an assessment order within the prescribed limitation period. Where the registered person furnishes a valid return within thirty days of service of the assessment order, the assessment order shall be deemed to have been withdrawn, but liability for payment of interest for delayed payment and for payment of late fee continues.