For transaction processing, wouldn't a 1-3% margin be about the maximum achievable? That would translate to about 15 million dollars in potential profit against a 730 million loss.
I think Klarna can charge slightly higher fees to merchants (it looks like they charge up to 6%) as sales increase with buy now pay later. Also they probably think they can make money off of servicing the debt (late fees, interest on balances).
Per https://www.klarna.com/international/regulatory-news/klarna-... their volume was $80B last year, 1.4B was their cut (1-3%) aka revenue, and the profit would depend on bunch of things such as their marketing/sales spend, wages, debt servicing, etc and most likely has been negative.
Edit: not "most likely" - apparently they lost to the tune of $700M