Create a Klarna payment session, present the widget, and manage the order lifecycle

Steps

1. Create a session: POST /payments/v1/sessions with 'purchase_country', 'purchase_currency', 'locale', 'order_amount' (minor units), 'order_lines' array (name, quantity, unit_price, total_amount, tax_rate), and your merchant credentials via HTTP Basic auth.
2. Use the returned 'client_token' to initialize the Klarna Payments JS SDK on the frontend: Klarna.Payments.init({client_token}) and then Klarna.Payments.load() to render the Klarna widget in a container div.
3. When the customer clicks your 'Buy' button, call Klarna.Payments.authorize() — this handles Klarna's approval flow (BNPL decision, ID check) and returns an 'authorization_token' on success.
4. Server-side, place the order: POST /payments/v1/authorizations/{authorization_token}/order with the same order_lines and amounts used in the session — amounts must match exactly or Klarna will reject the request.
5. Capture funds when you ship: POST /ordermanagement/v1/orders/{order_id}/captures with 'captured_amount' and 'order_lines'; partial captures are supported for partial shipments.
6. Handle post-purchase operations (refunds, cancellations, extending expiry) via the Order Management API at /ordermanagement/v1/orders/{order_id}/.