NACHA rules say that a consumer's stop payment on an individual transaction expires when the payment is stopped and returned. However, if the transaction is a pre-authorized EFT from a consumer account, it falls under the stop payment rules of Regulation E, which does not have any expiration provision.
As to whether your customer's stop order applies only to one entry or to multiple entries, that's up to the stop order that your customer issues and you accept. If you undertook to stop all six transactions, that's what you should do. But you would be correct if you insist that an individual stop order address an individual transaction and not a series. I won't address the fee issue.
Then there is the matter of whether the customer actually intends that his or her authorization be revoked. If that's the case, you should hew to the language of the comments to section 205.10(c).
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8