Regarding Interest charged by supplier on delayed payment during Automobile Crisis and Corona period

Dear Kaanoon team, I am Vygnesh Rao K B working for Icon Colour Coats(Bangalore) as Accounts Manager.Our Supplier named Indian Diecasting Industries(Aligarh) started to supply from August 2019 and he has supplied till January 2020. Till March 2020 he has supplied material worth Rs.78,98,920/- and we have made payment of Rs.38,72,220/-. in 2020-21 he has not supplied but we have cleared his dues fully on Sep 2020 after getting balance confirmation from them. But on [deleted] they are asking for Interest at 18% on delayed payment on every single bill and demanding extra Rs.13,50,000/- from us. We need to issue legal notice to him regarding this. Please Note : 1) He has issued bulk lots to us even when we did not have Schedule. 2) We have not sent any formal or informal Schedule for him to send the materials in such bulk lots. 3) We have paid the dues only after getting balance confirmation through mail. 4) At the time of final settlement he has not demanded any Interest. 5) We have not got any formal or informal communication regarding the Interest demand till 3rd of October 2020. Please help us with the process.