If you have a Returns Accepted Policy on your listings you can accept the return for the reason ' does not fit' but the buyer pays for the return.
If the buyer opens the wrong case , that is, one for item not as described, you are left with paying for the return, then trying to fight your corner with eBay proving the wrong case was opened, and you have lost a postage payment.
Not an easy route, I'll forewarn you !
@o-crezi