Ask HN: What are the low fee options to process Debit Card or ACH in mobile app?

I'm not sure I'm putting it right, maybe that's why I still haven't found an answer yet.

I'm looking at how Venmo works. Say if I want to make a clone, I can't seem to find a service to accept user's debit card or bank account for low or no fee. The options I have found so far are Stripe, Plaid, or BrainTree.

Currently Venmo charges 3% for credit card, and free for debit card or bank account. I wonder how they can make it free. Whether they are using an in house payment system or using paypal's.

If I were to build an in house payment processor, how can one go about to do that? Thanks!