Generates a hosted Stripe Checkout URL for a subscription's current DUE invoice (e.g., a renewal or upgrade invoice). Tanso automatically finds the first DUE invoice for the subscription.

Note: For initial subscription payments on in-advance plans, you don't need this endpoint — the create subscription endpoint returns a checkoutUrl directly. Use this endpoint for subsequent invoices on existing subscriptions.

Requirements: Stripe must be enabled on your account. Configure your Stripe integration in the Tanso Dashboard.

Already paid: If the invoice is already paid, the response will return successfully with an empty url field.