Package never arrived.. 17 days after it was marked delivered

New seller here, getting my feet wet and ran into my first snag. Was selling a good quantity of an item until sell out, and now I am finally out. Not sure where to go with this report from customer that it never arrived 17 days after marked delivered. Just trying to figure out how to be proper. Its only a $15 refund but if everyone did this I would get wrecked.

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Any tips? Do I still keep the shipping cost or is it full refund?