charged deffered interest due to their sytem error

EDGARS is useless i made 2 purchase in October 2019 and one purchase was reflected in my account in November and i settled the account in full. i kept enquiring about the second purchase and i was told its not in the system. in December the amount was included in my statement with interest. In January Edgars charged interest twice in my account when i called the customer line i am told i am being charged for the purchase made in October they are catching up on the interest. whose fault is it that the amount wasn't charged on time. now i must pay for their incompetence. They outright refuse to reverse the deferred interest charge and they giving me excuses on closing my account. i am done with Edcon. i want my account closed.