We continue to have customer requesting copies of checks which they have sent into a lockbox for their credit card payment. Our bank converts these payments to ACH. Are we obligated by law or regulation to honor these requests if they are not disputes?

My response assumes your bank is the credit card issuer and the payee of the checks in question. If you have any obligation to provide your customers copies of checks that you have converted to ARC entries directly, that obligation should be found in your agreement with your customer, or perhaps in state law (probably not the case in Delaware).

You do have an obligation under NACHA rules to retain a retrievable copy of the front (the back is optional) of such checks for two years, and provide a copy to the originating depository financial institution (ODFI) in time for them to get it back to the RDFI within ten days of a request from the RDFI.