Negative Feedback mima refused to pay custom fees, insisting on money back

I sold a Dolls Chateau Charles head to mima (located in France). Mima is now asking for their money back (including the shipping cost to them), indicating that they do not wish to pay the custom fees. I have let mima know that my sales thread clearly stated that all payments are non-refundable. I received another email from mima, asking once again why I could not refund them $100. Again I stated the terms of the sale and let them know that it was not fair of them to ask me to refund them payment based off their refusal to pay custom fees.