Penalised for paying installment in advance!

I have been a loyal Truworths customer for more than 20 years and have consistently been on time paying my installments. I fact, last month I paid an installment and later on in that same month I paid a second installment because I had some surplus money. In other words I paid way ahead of time. Instead of Truworths recognising that I had paid the installment for the next month and the month after that, they demanded that I pay the installment again. I received an "overdue" statement per e-mail and via SMS and they slapped on interest. I wrote an email to *** with all substantiating evidence that I was AHEAD of my payments (I have 6 months to pay off). I received 3 phone calls today. When I explained my situation, I was told by the Truworths consultant that I should have notified Truworths that I was paying ahead. Why should such a fantastic customer like me have to waste my time assisting Truworths with their accounting? I was so furious that I decided to pay the remainder of my balance (excluding the interest) today and ask Truworths to close my account. I received the third phone call still insisting I was in arrears. I told the consultant that I had paid the rest of my account at 13H33 and I now wanted a statement showing a R0 balance and that she was to close my account. She simply said OK. No apology and no regrets that Truworths was losing a customer with a great paying record. What infuriated me was that after this phone call, I received a SMS from Truworths saying: Thank you for making a payment arrangement on your Truworths account. Remember to pay the agreed R*** by 2024/05/09 to avoid further arrears. I am furious! First of all I have NEVER been in arrears, in fact I paid 2 months ahead! Secondly, the SMS insinuates that I was guilty and they are now reminding me to pay. I have already paid the full balance because it was MY CHOICE to settle my account even though my last installment is due 1 July 2024! If I am treated with such disrespect, I would hate to know how a customer is treated who really did miss an installment. Utter harassment from Truworths!!!