Don't see how ACH can work since the checking account (ACH source) is not really related to the SPECIAL $75/year debit card. A regular debit card on the same account doesn't generate Delta miles. Thus, the card drives the airmiles, not the checking account. This behavior has been confirmed through use and statements from the reps. They affirmed that the card HAD to be used for a "purchase" to get the miles and that providing the bank routing info instead via bill pay or ACH specifically didn't generate miles.
How did you get the debit card number associated with ACH transaction please?
By far the biggest benefit has been high ticket "purchases" like paying taxes or other expenses where the credit card fee is a percentage (2-3%!) whereas the debit card fee is capped at less than $3 flat rate for up to the $35k per debit card purchase available with this card. Since this card generates the same airmiles on Delta as Amex and Amex is a lot less accepted, this card also works well for places that don't accept Amex.