SunExpress Airlines Extra Baggage Fee Issue and Lack of Refund

One day before the flight, I purchased additional baggage allowance. However, at the airport, I was told it was not registered, and I had to pay an extra baggage fee of €200. I spoke with customer service, and they said the payment was made one hour before the flight. I sent all the documents and created a claim file to get a refund for the money unfairly taken from me at the airport. Despite a month passing, I have not received any response.
It is very disappointing that such incidents have been happening recently with the airline we have preferred for years...