Is there also no fee for merchants? I thought business accounts are usually not as cheap as (or free like) consumer ones, some (all?) iirc pay per transaction or have tiers

It's also really hard to interface with. Afaik, I can't simply get an API token from my bank and send 2-cent transactions to pages I read if they'd publish the IBAN as part of an HTTP header or meta tag for example. Nor do I know that my bank would be happy about a thousand tiny transactions each month