Those are usually interest free services. The company selling you the product pays the fee. Which means the fee is baked into every product.

But also it’s a way to increase sales, otherwise they wouldn’t offer it. Not sure why the fuck they’re offering it on fast food in the US though, it’s meant for stuff that cost a bit more than you could afford right away in one payment but won’t make you destitute to pay in 3. E.g when I bought a bicycle, I bought a slightly nicer one than I had spare cash for, but paid the exact price over 3 months, not a cent extra. Not with Klarna, but similar service in my country.