Here's the thing, though. There's no interest charged to the customer. I think Klarna makes its money just because companies pay them money for integrations and for the ability to advertise that customers can buy now pay later and such. And at least in the case of my company's integration with Klarna, Klarna takes all the risk. They're lending customers money and hoping the customers pay it back. My employer gets the money up front and isn't out any money if the customer doesn't pay.

Your company pays a transaction fee just like with a credit card. Except it's usually roughly twice as expensive as a credit card. This is what allows Klarna to take on all that risk, generally. For your company this is essentially a marketing expense. Offer a convenient way to pay in return for a few percent of the transaction (3-6% + a fixed fee, $0.30 perhaps).

Klarna generally partners with some financial firm to finance these short term loans, and they use the merchant fee to pay interest. These can be as high as 25% APR. It's a high risk loan.