Threatening customers with legal action within a month after purchase and payment of installment

I have a Truworths account on a 6 months interest free plan for the past 18 years since 1999. For quite some time I didn’t buy anything at Truworths so my account was on zero. Then I made the mistake to start buying there again. I bought a pair of denim jeans on 27 Aug 2017 for R599 and another on 28 Aug 2017 for R599. On 27 Sep 2017 I credited the pair that I bought on 27 Aug 2017 and paid R100 on my account. On 08 Oct 2017 I bought a pair of shoes for R360 and credited it again on 19 Oct 2017. But then Truworths start sending the following sms’s to my mother’s cell phone. Where they got hold of her cell phone number I don’t know. 27 Sep – Thank you for the recent transaction on your Truworths account. Hope to see you in store again soon. 10 Oct - Get immediate credit. Pay R328.00 or set up debit order. 13 Oct 13:06 – your TRUWORTHS account remains overdue. Pay R328.00 TODAY. Non-payment is affecting your credit status and may result in handover. Tel: ********** 727. 14 Oct -11:31 – Avoid handover for collections of your TRUWORTHS account by paying R328.00 TODAY and prevent a negative credit profile due to non-payment. Tel: ********** 727. 16 Oct – 14:42 – Your TRUWORTHS account is arrears due to non-payment. Pay R328.00 IMMEDIATELY to avoid further action and to risk losing your credit. Tel: ********** 727. 18 Oct -14.40 – Settle the arrears on your TRUWORTHS account to reinstate your credit IMMEDIATELY. Pay R328.00 TODAY to stop the handover process. Tel: ********** 727. 20 Oct – 12:05 – To stop HANOVER to a debit collection agency TRUWORTHS requires URGENT payment of R328.00 TODAY. Tel: ********** 727. 20 Oct – 15:31 – a reminder that your TRUWORTHS account is overdue. Please PAY R328.00 as soon as possible to bring it up to date. Tel: ********** 727. 21 Oct – 11:51 – your TRUWORTHS account remain in arrears by R328.00. IMMEDIATE payment will avoid handover to a collections agency. Tel: ********** 727. 23 Oct – 12:20 – an outstanding amount of R328.00 is preventing you from accessing your credit at TRUWORTHS. Pay TODAY to stop handover of your account. Tel: ********** 727. 25 Oct – 12:56 – Your Truworths account remains in arrears which is in breach of contract and may be identified for handover. Please pay R328.00 IMMEDIATELY. Tel: ********** 727. 27 Oct – 12:51 – TRUWORTHS is offering you a final opportunity to keep your credit facility by paying R328.00 TODAY. Only URGENT payment will stop handover. Tel: ********** 727. 28Oct – 09:50 – You have 48 hours to pay R328.00 to TRUWORTHS to avoid handover to a debt collection agency and to retain your credit facility. Tel: ********** 727. 30Oct – 13:41 – You are in breach of contract with TRUWORTHS. Pay R328.00 TODAY to prevent losing your credit facility and stop further action. Tel: ********** 727. On 01 Nov 2017 I paid R100 on the account and it showed that there is R28 still outstanding, so I paid that as well. So my Current Balance showed to be R418.11. This Balance didn’t make sense to me because I actually bought only one pair of jeans for R599 and paid in total R228 on the account and as I am on a 6 months interest free plan the Balance should have been only R371 so why is my Balance now suddenly R47.11 more. So I called Customer service and talked to Pumza and asked her what this difference is because as far as I know the card protection fee is only R18 per year. She said no the card protection fee is now R28. I then asked her but why is there then still a difference of about R18+ and she told me she really doesn’t know. I became very upset and asked her why they send me all these threatening sms’s within a month after I did pay. Do they want me to pay the account off in two months instead of six. She couldn’t answer me on that and then she said that the Balance difference is interest they charged because I credited to late. That upset me even more because I did credit within 30 days according to their rules. I also asked her to email me my statements but that also didn’t happen. Why do they threaten me with legal action? Is it because I did pay or because I started to buy there again? What I want from Truworths now is that they set my account right be removal of this so-called interest and stop sending me any correspondence to my mother’s phone.