Searching for a Peer to Peer ACH API

Hello all - longtime listener, first time caller - forgive protocol transgressions.

I'm working on a weekend project that is essentially a super niched down Point of Sale / Account Management platform. I've done a decent bit of customer development and have something along the lines of a product built, but I'm stuck at payments.

Backstory: My users have customers, many of whom have in-house charge accounts with my users. Totals vary, but I'd say they run up a $350-$500 tab each month. My user keeps track of this in whatever archaic way suits them and bills them each month, almost exclusively by printing a paper invoice and mailing it. Their customer in turn cuts a check and mails it back. I'd like to bring that into 2017 by simply adding ACH function to the mix. Sure, they could roll their own, but we're talking about a non-technical group of small business owners here, they'd buy it if it was done for them but they haven't even considered the prospect of doing it themselves.

Problem: As I mentioned, I've got what could pass for a product built out and working, minus payments. I need a ACH API provider (bonus points if they offer Credit Cards too, as this is part of the "retail sale" use case for my software) that will either allow peer-to-peer ACH transactions via API, or equally as good, allow me to collect on behalf of my customer and distribute via a second ACH transaction.

Still with me?

I would use Stripe as I have in the past, except they charge a percentage (0.8% up to $5) for ACH processing and require the end user to validate their account through Plaid or confirming micro transactions. This percentage-based pricing model is no big deal if my average transaction size were smaller, it'd be literally pennies. But, I'm dealing with $300-$500 transactions so transaction fees would be $2.40 - $4.00 - my customers would rather stay in their old ways and print, mail, and receive a check regardless of the inefficiency. If that weren't enough, the end-user validation piece is another hurdle that is probably completely unnecessary for this use case.

Other solutions I've entertained:

Dwolla - They've got an incredibly robust looking API for just this sort of thing, sorta. Problem is, the Access API starts at $2000/mo, far above my budget for my basement-based weekend project. They have a consumer grade solution, but it looks like there's no way to create the functionality I'm looking for without requiring my user to create an account on Dwolla as well.

PaySimple - Flat rate $0.60/transaction but requires my user to create an account and go through their approval process as well. Interestingly, they offer a 30% 'bounty' on their net transaction fees, which brings the price down. I could easily factor the $0.60 into my pricing, but a greater concern is that my user would be required to create an account with them. In conversation with one of their reps, they metioned they have clients in an market adjacent to my target market so the concern that my payments platform could rob my customer is not unfounded.

Not working against the tide - I could not offer ACH - i.e., my software would do everything up to the billing, but require that users print and mail an invoice like they're doing today. I don't want to, because I build things. I could even integrate with Lob so that the invoice is mailed. But again, I don't want to do this because I build things and I would love ACH payments to be automagically deposited in my user's account.

So community, what say you?

• Do you know of a terrific, low flat fee ACH provider who allows this peer to peer (customer of my user to my user) style payments without requiring everyone sign up for a different service?

• Or should I get over myself and make it so that invoices are still mailed, or done via API?

• or go raise a holy crap ton of money and build a ACH payments platforms that allows software developers to enable payments between two other parties? (don't pick this one. )