RCS incompetence causes me grief

This company's incompetence is causing me grief for months now, been calling me non stop and setting lawyers on me after I overpayed them for over 3.5 in 2019 and they made me spend the money without planning as they told me that their system was not allowing such, again now I sit with the same challenge as they claim I owe them for some reason payment of 1000 I paid on 9//2020 is not reflected on their system, I feel abused and harassed only this time I refuse to overpay them again Regards Lumka Ndawo ***.