Just don’t miss payments then. They make money from sales cut not from fees. E.g. Amazon will take 6% loss to get a sale giving Klarna 3% and losing 3% to inflation.

Looking at Klarna for example it’s 5$ late fee and then it gets reported to relevant authorities (credit, collections etc.) and they just block you.

It’s not that tricky like payday loans or similar. The issue at hand is really just people buying more than they can afford because they are not charged instantly.