How to respond to buyer message. Package undeliverable

I sell only with FBA and I received the following message from a buyer this morning

This book was “undeliverable”, not sure why. Can you please let me know the original price (I ordered multiple copies of this book) and when I can expect a refund

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I believe they need to be contacting Amazon not me, but part of me just wants to refund them and move on and not risk getting bad feedback. Any advice?