Amazon policy when buyer does not use pre-paid returns label

So I sold a hat to a customer who returned it as it not fit.
I have to offer free returns on this item. But customer did not use the pre-paid returns label given to them by Amazon - instead they paid for the return themelves using a different higher priced service.
So I goto refund - but Amazon wont let me refund unless I enter a cost for the return label.
As the price they paid is on the label I have entered this value so I can process the refund.
But what is Amazon policy on this - could I or should I have entered the lower cost of the returns label if they had used the one provided by Amazon or do I have to refund the actual cost to the buyer?
Im just concerned that one day a customer returns using Special Delivery or something costing a lot more.