Customer received refund without asking

A worrying thing has happened today. We’ve received an email from a customer through our website (assuming searched by company name from Amazon order). She advised the order (FBA order) she placed with us has been refunded by Amazon. She is happy with the product, it is in use and has had no contact with Amazon about the order or requesting a refund. It has literally automatically just happened. Check the order and yes it’s showing as refunded. She wanted to pay us as feel it’s wrong to receive a refund when she has the product (love these customers!). I can sort this out with her but the question is, how many times is this happening when the customer isn’t contacting us?