Affirm doesn't list their B2B pricing on their website, but credit cards typically charge up to 5% of the total purchase price in processing fees. It wouldn't be hard for a challenging upstart to make a solid business that just charges less. That's probably not the primary revenue driver for most BNPL companies though.

You've also seen credit card companies change their offerings to compete with the BNPL model. One of my credit cards has a "plan it" feature for large purchases, which allows you pay it of in a shorter period of time for a lower interest rate and without incurring interest charges on your month-to-month purchases.