Defective or not as described merchandise is not covered by Reg E. While you do have an obligation under VISA rules to assist the cardholder, you can require that the cardholder meet all of the conditions as set forth in the VISA rules.
An Issuer must credit its Cardholder's account for the amount in dispute, whether or not a Chargeback was initiated, if the dispute involves an Electronic Commerce Transaction and the conditions are met for any of the following Chargebacks:
Reason Code 30, "Services Not Provided or Merchandise Not Received"
Reason Code 41, "Cancelled Recurring Transaction"
Reason Code 53, "Not as Described or Defective Merchandise"
Reason Code 83, "Fraud-Card-Absent Environment"
Reason Code 85, "Credit Not Processed"
Since the cardholder failed to meet the requirement to put their claim in writing, you are not obligated to investigate or to provide them credit. Reverse the provisional credit until they provide the needed documentation for you to file a chargeback.
I can do all things through Him who gives me strength. (Phillipians 4:13)