Kiyafet Sepeti Order Return Problem

I purchased a dress from Kiyafet Sepeti, and it arrived defective and one size too big. Since it was a gift, I missed the exchange period. However, after providing pictures showing the defects, they issued a return code. When I checked the next day if the return code was still valid, despite the courier company (MNG) being closed at 5:00 PM, the Kiyafet Sepeti representative told me the return was impossible and they should not have given me the code in the first place. They insisted there was a mistake, and they couldn't accept the return under any circumstances. Now, I'm stuck with a defective dress worth ₺5,000, paying the price for poor communication within this company. This was my last purchase, and I strongly advise against anyone shopping here. The dress quality is mediocre, the lining is coming off, and they can't even handle a simple exchange.