Not Fair Business Practice.

Unfair Business Practice. I opened an account with Cape Union Mart which was approved, a credit limit given and issued a Cape Union Mart Card. This was 21 March 2020. I Purchased goods to the value of R2596 (R4500 Credit Limit). Installmant = R332.28 per month. I made a Payment R500 on the 4th April. 16 April I receive a Statement indicating that a payment of R332.28 is due by 1 May. I contacted them in this regard as to my Knowledge the statement should reflect R0.00 due 1 May as I have already made payment for the month of April. Not according to them. Because I made Payment before the Ledger date, this is not recorded as a installment. The amount paid is set off against the total outstanding amount. ***!!!!!!!!!!!!!!! Therefore in order not be in arrears post the 1st of May I have to statement Installment. in other words I have to make a double payment?