Randy's right. ACH authorizations can be revoked, and there is a right to stop payment under Nacha rules, but neither revocation nor a stop payment has any effect if the transaction has already posted to the customer's account. In my experience, it's not likely for the transaction to hit the account after the package has been delivered.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8