Looks buggy to me. I created a test invoice and specified that it should be payable at a later date and only via ACH. I checked the preview link and the only option to pay was by credit card. I sent the invoice anyway and the test user's credit card was immediately charged.
(Engineering Manager at Stripe Billing) Ah - sorry about that, a last minute fix caused a bug. This is now fixed in production. Thank you so much for reporting!
1) Send a user an invoice
2) The user copies the auto-generated bank details and manually initiates a transfer from their bank?