Cybex Aton5 Infant Car Seat Missing Part Complaint

I'm reaching out to express my dissatisfaction with Opera Baby Store, a baby store located in Istanbul, Turkey. I bought a Cybex Aton5 infant car seat and Mios stroller from their Ataşehir branch last month.
The salespeople at the Ataşehir branch spoke highly of the Cybex Aton5 and Mios and informed me that I should bring the box to the Opera Suadiye branch for the car seat to be installed in the stroller.
When I went to the Suadiye branch with the box, I found out that the belt buckle was missing. I reached out to Opera right away to let them know about the missing part, but they haven't been very helpful.
I've been dealing with this problem for two months, but Opera has not sent me the missing part or refunded my money. They're making me pay ₺1,500 for the missing part, even though it was their fault it was missing.
I'm really let down by Opera's customer service. I've been a loyal customer for many years, but this experience has made me reconsider shopping there in the future.