Partial Refund on an FBA order

We had a customer complaint regarding a products box having already been opened before it arrived to her.

We offered to reimburse 50% of the cost and she kept the item or to return for a full refund.

The customer chose to keep the item and accept a partial refund.

How do I now process this, the item was fulfilled by Amazon so does the customer need to make a claim through Amazon or can we do it ourselves to save time and hassle the customer any further?

All the info I’ve seen says to go to the manage orders page and click on the refund link in the actions tab but that is not available on my screen.

Any help would be greatly appreciated.

Thanks

Alan