Charging interest on closed accounts

I write this review with anger. I had an RCS card since 2015. I stopped using the card end of 2016 and have been just paying installments every month. Last month i paid the balance in full. Called in 2 days after to confirm that i am closing my account and it was confirmed that account is fully paid and closed. Now i get a statement that i have an installment of R73, How? If the account is closed? Sent message on their Facebook page, so ridiculous their response and still waiting for someone yo contact me since last Friday. I am not going to pay anything as i have closed that account.