Mytrip.com Charged Claiming Credit Card Fee!

I bought a ticket from mytrip.com at a specific displayed price, but after purchasing, I was surprised to see an additional amount charged, which they claimed was a credit card fee. However, the website clearly showed that paying by credit card had 0 fees. This is unethical, and I do not accept it. Please help me escalate this issue and file a complaint, as their customer support said they could do nothing about it. Thank you.