Biletsepeti.com Charged $450 for Flight Date Change Despite Free Change Option

I purchased a ticket through Biletsepeti.com with the right to change it without a penalty. However, when I tried to change the date of my economy class flight from the USA to Turkey, I was charged a $450 price difference. If I were to buy the flight from scratch, the total would be $720. Despite multiple emails requesting an explanation, I received no responses. When I asked for clarification, I was simply sent a link and told to pay the fee without any explanation of what the price difference was for. This lack of transparency and poor communication is unacceptable.