FNB does not want to refund me for their mistake

Both my husband and myself cancelled our life insurance with a certain provider. Because the bank was the beneficiary (home loan purposes), they had to send through certain documents to this provider. Upon assuring me that the documents were sent, the provider then debited my account even though the policy was cancelled. When I requested feedback from the provider, they told me that FNB sent the documents to the wrong email address and that I would have to seek the refund from them. Fair enough. That was the beginning of last month and FNB sent the documents at the end of June. I am still waiting for a resolution since last week when I finally obtained the email address of the person who had made the error. This FNB consultant advised me to email his manager if no resolution was forthcoming, I did that. I told them to refund the funds by 3pm last Friday (31 July), no refund and no apologies. I sent another mail and received a response stating that the mangers were not in office and there was still no resolution. I responded to this mail stating my shock that no-one could assist me but again, no response. Why should my husband and I pay for FNB's R 800.00 mistake?