Oh, I think the BP provider must complete the investigation. It's definitely a Reg E thing. But, once the BPP has determined that the payment went to the correct place, that's as far as the BPP is required to go.
That said, particularly if the payment went to the payee electronically, the BPP would have a lot easier time dealing directly with the payee's A/R department than the customer would. So a quality BPP would attempt to stand in for the customer in following the dispute through to the point where the payee figures out what went wrong and corrects it.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8