So the processing fees (called interchange) are capped at a $0.21+0.05% low rate for most banks, but very small banks and credit unions are exempt from this cap

So the processing fees (called interchange) are capped at a $0.21+0.05% low rate for most banks, but very small banks and credit unions are exempt from this cap, and charge ~2% similar to credit cards. Fintechs always partner with a small bank to issue their cards and split the revenue. So the thing is, most of their customers are going to be connecting a debit card from a large bank (they don't allow credit cards as a funding source). A few will come from a Durbin-exempt institution like a small credit union or a community bank (including one issued by another fintech/bank partnership such as Chime). But on the whole, they will mostly be paying the low interchange and charging the high interchange. Since the new cards are considered credit cards, not debit cards, they won't have to worry about Durbin amendment at all.