How to handle unexpected customer refunds without notification?

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we encountered a very serious problem and need to consult. We have a customer who recharged $710 for text message sending and used up the balance. But later we found out that this customer refunded all the $710 in Shopify, and we did not receive any refund information in our system, and we did not receive any email reminders, which caused us to lose this money in reality. We would like to ask why customers can refund at will without notifying and negotiating with us? How do we appeal?