Confirm a checkout session to finalize the purchase, create a contract, and handle payment processing.

Confirmation modes (confirmation):

• charge_immediately: Redirect to payment gateway to collect payment now; with redirect=true returns 302 Location, otherwise 200 JSON.
• charge_when_due: Do not charge now; requires an existing saved payment method; creates contract and schedules payment.
• no_payment: Skip payments (free/internal flows); creates contract without charging.

Common use cases:

• Self-service checkout: Use charge_immediately with redirect=true to send users to payment gateway
• Enterprise deals: Use charge_when_due for customers with payment terms
• Free plans or trials: Use no_payment to create contract without payment processing
• Guest checkout completion: Creates customer profile and associates with contract

Behavior & requirements:

• Only 'open' sessions can be confirmed
• charge_when_due requires at least one saved payment method
• Guest checkout creates the customer profile during confirmation
• redirect=true returns an HTTP 302 redirect for immediate collection flows
• Successful confirmation creates a contract in the system
• Payment status transitions from not_started to pending, paid, or no_payment_required

Related endpoints:

• POST /checkout_session: Create the session to be confirmed
• GET /checkout_session/{id}: Check session status before confirming
• PUT /checkout_session/{id}: Update details before confirming
• POST /checkout_session/{id}/cancel: Cancel instead of confirming

Errors include invalid status transitions, missing customer/payment methods, and contract creation failures.