First Citizens Bank Account Or Service Complaint

First Citizens Bank account or service department,

Checking account Deposits and withdrawals North Carolina

I made an online purchase using my debit card on - - 2015. The merchant did not have one of the items I ordered in inventory, and said they issued a $10.00 credit for the item. I have never received the credit. I talked to the merchant and they said everything has processed on their end, and there was nothing more they could do, and to call my bank. I called my bank, and they said there is nothing they can do. My understanding is that the bank has an obligation to investigate any reported error, however they refused to do so. They said my only option is to return the part of the order I did get and get a refund for the entire order, or dispute any incomplete refund from there. This does not make sense. I feel the bank has an obligation to investigate.