Bad service

I have always thought it’s the best ,until I was charged twice for my Laggages and when I lay a complaint they take long to get back to me ,An Agent called Mercy Van Heerden always tells me she will call me regarding feedback,I always need to send her emails regarding my complaint,the last email was send was that I was using my card to do payments for someone else and not even on the day I was at the airport,I’m not sure what to do or where to go now as when I call them they tell me they will get back to me ,please assist as to where can I go or how can I take this further???????Not happy with their services