Sometimes, a customer wants to make a return for a reason that is obviously, not stated under the return reasons you have customized or have available on your portal.
You could then simply add a return reason named "Other". This way, you have the existing comment field for each item in the return that is visible and where the customer can enter text. Return Reasons can be found and added under Return Settings >> General >> Return reasons. You could enter the reason in a new line, if you have added other reasons previously (see image below).