Conyac で依頼された翻訳結果を公開

I refunded $200 to a buyer in accordance with their previous claim. However, I received another claim again about the product after about a month, which is on October 28.
Why did they submit the claim after 24 days? If the product was broken from the beginning, they would have said something about it when they submitted the first claim. It is incomprehensible that they did not notice the defect of the product for 24 days.
The other problem is that the buyer continuously threaten me to write a negative feedback about me. Even if I refund the money again, they may submit the third claim and threaten me again. It is possible for them to threaten me over and over again with regards to my feedback.
Please tell me the best solution.