How should I handle an incomplete online transaction refund in South Korea?

Question

Hello. I am reaching out for help because I encountered a problem with an online transaction I made in Korea.

[Summary of the Question or Situation]  
As a foreigner, I purchased a VPS server in Korea, but the seller did not provide the server by the promised date. I requested a refund, but received no response, and only today was I informed that the server is ready. However, I no longer need it and would like to receive a refund.

[Key Context]  
The transaction amount is approximately 600,000 won, and I know the seller's company name and account number. I communicated via KakaoTalk and made the payment through KakaoPay transfer. I plan to file a complaint with the bank and the Financial Supervisory Service.

[Request for Help or Opinions]  
I would appreciate advice on how to obtain a refund in this situation. Additionally, I would like to hear the experiences of others who have faced similar issues. How can I hold the seller accountable?