Oppo Authorized Service Overcharging

I wanted to send my Oppo A96 phone, which was under warranty, to the technical service because it was turning off and on by itself. I reside in Ankara, but they informed me that the service center was located in Istanbul, and I needed to initiate a service request. I registered the issue with the Oppo technical service, under the name Byfix. They informed me that the device's motherboard needed to be replaced. They sent it back to me but demanded a fee of 890 TL to be paid at the door. This practice is causing unnecessary trouble for Oppo customers. Why should we pay for a phone that wasn't even fixed? Since when did it become necessary to pay a fee to send a phone to an authorized service center? I am determined to take this matter to all relevant legal authorities.