Here’s what I thought, feedback welcome.
Klarna works this way:
The merchant pays Klarna. Then Klarna pays the merchant the full amount then takes it as a responsibility to chase the customer for the full money. So Klarna is paid, and provides a credit and the chasing of the customer. Because a payment for credit is riba, Klarna is certainly haram. If we’re a customer and not a merchant, so that we’re indifferent about the credit, it’s still haram, because how Klarna works is well-known, and it’s forbidden to witness riba or to encourage it, the reason to avoid it remains.
The other view of Klarna is that they are being paid and they allow a delayed payment, the same ruling applies.
I found a fatwa regarding profiting from guarantorship: It is not permissible to accept money in return for acting as guarantor for someone - Islam Question & Answer
As for PayPal Pay in 3, they charge no more for this kind of credit in comparison to an instant payment. If we consider it intention-wise, it’s halal since no charge is taken for installments, but if we consider that they provide installment-payments and they charge a fee, it’s haram. Besides, the reason they encourage installments is to increase trade volume hence the collection of fees, which seems very suspicious to me. Hence, it seems better to me to avoid PayPal Pay in 3. Allah knows best and please let me know if I’ve made a mistake.