Buy now, pay later online payment provider Klarna began trading on the New York Stock Exchange on Wednesday. Its IPO was priced at $40, which valued Klarna at about $15 billion, but opened at $52 per share in its debut.

"To me, it really just is a milestone," Klarna's co-founder and CEO, Sebastian Siemiatkowski, told CNBC on Wednesday . "It's a little bit like a wedding. You prepare so much, and you plan for it, and it's a big party. But in the end, marriage goes on."

Meanwhile, just a few days before the IPO, Klarna, which was founded in 2005 , told employees that it is joining Microsoft , Target, and other companies in mandating a return-to-office schedule (RTO) — in Klarna's case, three days a week in the office starting September 29.

Sebastian Siemiatkowski, chief executive

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