The customer is welcome to contact the merchant, but as you know you can’t require it. I know you aren’t requiring that, but make sure your procedures and the documentation reflect that since it sounds like that is what they will be doing. As for the bank, I would follow normal procedures once I received notification of error from the customer. A conversation with the customer regarding your requirements and process would be good customer service and might help them understand what you will be doing to help them. Reg E states:
(b) Notice of error from consumer. (1) Timing; contents. A financial institution shall comply with the requirements of this section with respect to any oral or written notice of error from the consumer that:
(i) Is received by the institution no later than 60 days after the institution sends the periodic statement or provides the passbook documentation, required by § 1005.9, on which the alleged error is first reflected;
(ii) Enables the institution to identify the consumer’s name and account number; and
(iii) Indicates why the consumer believes an error exists and includes to the extent possible the type, date, and amount of the error, except for requests described in paragraph (a)(1)(vii) of this section.