If the customer authorized the transactions, then they really have no claim under Reg E. Reg E does not cover disputes with respect to quality or receipt of goods. If the transactions were truly POS, i.e. not run through Visa/Mastercard, then the customer is on his/her own to resolve the dispute with the merchant. If the transactions were Visa or Mastercard, then their rules apply. I am not familiar with all of their rules, but there is probably some provision for merchants and processors to provide timely information, just as you are required to provide timely resolution. You are still ultimately liable to your customer but might have some recourse against those parties that did not comply with the rules, if that is the case.
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I've just writed a wrong.