Incorrect last debit amount.

I bought my son's laptop from them 3 years ago, now end of August was suppose to be my last amount to be paid. They were suppose to debit last amount of R279.04and they went ahead and debited R389.14 How will i support them going forward or how will i recommend them to other's if they take what they not suppose to take? Every cent matters to me but they ****** me after being so loyal to them and not even missing one payment. I am not happy with them. If they still need my support going forward, they must reimburse me extra that they took. I sent them an email and no one cared to respond.