To answer my own question:
To use ACH in AR, an 'EFT Services' account is required with Sage Payment Solutions, which requires the Sage 100-customer (Merchant in SPS' terminology) to submit an application and a few months of bank statements for approval. If the Merchant already uses SPS for 'Credit Card' service, approval should be quick. Sage Payment Solutions will perform any demos and any/all Merchant training that you would like.
Once the Merchant is approved, they can contact their customers and have them fill out an authorization form to allow Sage Payments to perform ACH pulls on the Merchant's behalf. The customer's routing and account details are stored in Sage's Vault, not in Sage 100, so the data is encrypted, secure, and PCI compliant. Then whenever the Merchant creates an AR/SO Invoice or Cash Receipt with an ACH Payment Type against an ACH-allowed customer, Sage Payments will be notified during the Journal Update/Posting to perform the ACH pull (there's a checkbox to stop the transmission to SPS, if desired).
There are no monthly costs or minimum fees anymore; it's simply Transaction Fees (usually ~20c each), Reversal Fee and Chargeback Fees.