Customer received wrong item

FBA? or FBM?
Did you send the item yourself and accidentally pack the wrong item?
If so simply authorize a return, preferably with a pre paid label, and once back fully refund.
Ask the buyer to reorder for the correct item.
You could send a replacement, but Amazon do not recognize replacement shiments so you may end up two items down if you opt for this route.
If FBA simply refer the customer to CS as that is what you pay Amazon fulfillment fees for.