repetition of calls from debt collector over a payment which is received by BMW financial services

Good day I am very unhappy with the BMW financial services. I have made a payment of R3500 to BMW Financial services for a payment of X2 on the 28-10-2020, unfortunately I paid it, using incorrect Reference number which belonged to my prior X1. I communicated with them on the 5th of November 2020. I even send them a letter also with a proof of payment to show that the amount is been paid. I received a call from one of the consulted to told me that they have received my letter and the proof of payment and the matter will be resolved as soon as possible and I will receive a call once the matter is resolved. The following is the letter which was send to this email address ***

I have mistakenly made a payment of R3500.00 to *** which is the old reference number which belonged to the x1 which I already settled. May you please allocate the payment made of R3500 to my correct account for X2 which is ***.

Since I never receive a call back from BMW financial services, on the 11-01-2021, I called them and they advise me to resend the letter to a different email address which is ***. on the 21 January 2021 I called the customer care at BMWFinancialservices and they told me that they can see the letter and letter is sitting in one of their managers table and they don't understand or know when they will attend to it. My problem is my credit scoring is affected by this; every week am receiving the calls from BMW debt collectors.

May you please help me.