Nedbank's disgusting service to a loyal customer of 40 years

I have been a customer of Nedbank card division for over 40 years and the one time I am not able to pay my monthly amount on time due to not being close to an internet facility and then paying 3 days late,they bill me with R300 interest. I contacted the professional banking division where I am a customer to be told that sorry they cannot do anything to wave the R300. I am a pensioner and an extra R300 is a lot of money. I am also a client that has never in 40 years defaulted on a payment. I am extremely disgusted in the treatment I received from Nedbank.