Swedish buy now, pay later firm Klarna made a strong NYSE debut, with shares jumping nearly 15%. Opening at $52, a 30% premium, the stock closed at $45.82. The IPO, raising $1.37 billion, is the year's largest. Klarna, founded in 2005, aims to challenge credit cards, offering 'pay-in-4' plans. CEO Sebastian Siemiatkowski eyes U.S. growth.