Any adjudication order or decision made or any summons or notice issued under this Act shall be served:

(a) by tendering the adjudication order, decision, summons or notice or sending it by registered post or courier service to the person for whom it is intended or to his agent; and

(b) if the adjudication order, decision, summons or notice cannot he served in any manner provided in clause (a), by affixing it on the notice board of the local Sales Tax Office.