Incorrect installment on account

It seems that this company does not know how to calculate installments or does not know what installments mean. My Woolworths account handed over to Nimble now has an installment of R7935, also the balance on the account. I have queried this by email and weeks later without any investigation or explanation, received an email that the ticket was closed. I have responded and have not received a response. They do not respond to emails, but can harass a person at work at all hours of the day. I just need the correct installment to reflect on my credit profile.