Free for individuals, but paid if you're the business it seems from what I've found? If that's the case, it seems to be the same as a card in the US and I wouldn't be surprised if some businesses try to pass it on like they do with cards.
[0] https://www.mollie.com/growth/wero-payment-guide#:~:text=How...
[1] https://www.bnpparibasfortis.be/en/public/faq/how-much-does-...