Rude and unprofessional service of a long-standing client

I was phoned by their head office by a very rude individual claiming that I still owe them money for my spectacles. This after I went to the branch to settle and pay the full amount myself 6 months ago. Phoning the branch didnt help either, as they seem to be dancing to the tune of whatever their head office says. Their excuse is the person who assisted me gave me the incorrect amount to pay. I have been a client for 20 years, and this is the last time I will visit a Mellins branch, to pay the rest of the account which they claim I am liable for. You don't tell someone that your service will cost a certain amount, make them pay that amount, and then months later make them feel like a ******** for not paying the full amount.