Settled account

My Woolworths account was handed over to them , I made the payment on the 4th December 2019 but they can’t cant give the paid up letter since they claim they can’t find any payment made even if I’ve sent them proof of payment worse case scenario my account is now handed over to other attorneys I’m being Harassed every day they are busy demanding the payment for something that was paid 3months ago meanwhile my my credit record is being affected .