Ask HN: What's the cheapest ACH payment processor for marketplaces?

You probably might want to start with the 0.5% Pay as You Go plan and as you go bigger, you can switch to a fixed-fee model. Once you switch to the other plans, you will be paying a flat ~$1 per transfer (both sending and receiving).

Handling fraud and chargebacks for ACH is more complicated compared to processing payments via cards. You probably should look into this before going live.

Source - Have built payment flows with ACH and cards for multiple marketplaces. Not associated with Dwolla at all. Just a happy customer.

EDIT:

Alternatives (that I have not tried) are Synapse - https://synapsefi.com/ach and Finix https://www.finixpayments.com/

For very low transaction volume your costs to set up the integration in first place may dominate the total cost, not the per transaction fee.

If you have high transaction volume you could shop around different payment providers for customised quotes.

For sufficiently high transaction volume it might be cheaper for your business to internalise some or all of the work to process the payments, but at that point you would need to be setting up systems to solve many of the problems that payment processors deal with.