Trendyol Overcharges Me on Sinoz "Buy 3 Pay for 2" Promo

I wanted to take advantage of the "buy 3, pay for 2" promotion on Trendyol for Sinoz's nourishing peeling cream and Pink face cream. Each item was priced at $5, and the cart also showed an "add to cart for $0" offer for a sunscreen. According to my calculations, I should have paid around $9 total for the 3 items plus $0 for the sunscreen — roughly $9 altogether — but my card was charged $17.
Today, about an hour after placing the order, I contacted Trendyol customer service via chat, and I was told something like "whichever item is cheaper will be the one discounted." But given that the screen clearly showed "buy 3, pay for 2" and "$0" for the sunscreen, I believe my payment should have come out to around $9 based on these promotion terms, and charging me $17 instead is a misleading practice toward consumers. My order number is 11334375228.
For this reason, I'm requesting a refund of the excess amount charged. Since I was charged $17 instead of the $9 I should have paid, I want approximately $9 refunded to my card, and I want the promotion terms corrected so they don't mislead consumers.