issue with return requests as FBM seller

Hi guys, I sell on Amazon as FBM seller. My products are customized. Customers have to select size in the customization. When they choose bigger size in the customization the price goes up. Customers buy the default one and claim they bought a bigger one. What can I do? I have the proof how much they paid and that they bought the smallest one yet they want refunds because wrong size was sent. Any help would be appreciated.