I can be wrong, but Shopify nor any of the other hosted platforms will have an option like that. ACH is something your payment gateway would use to settle your funds.
However, the closest thing that I can think of is an e-check payment. For example, the Authorize.net gateway allows customers to pay using an electronic check. But again the customer payment will first go to your payment gateway before it is settled to you.
I do not even know why you would want to have an ACH option for your customers, this is not popular at all. Credit cards and PayPal is the way to go. Good luck!
Yeah i can get this setup for you easily. I'll get a hold of you tomorrow and walk you through everything. Or if you prefer a particular time for me to contact you just email me at email@example.com