Cancels the order. Its status becomes cancelled, and no more payments are collected.

• You can cancel an order in any status except complete and cancelled.
• If the order is already complete or cancelled, you get 200 with the order unchanged. It's safe to retry a cancel.
• Upcoming scheduled payments are cancelled too. Nothing is charged or refunded. Payments already taken stay as they are.
• You can't undo a cancellation.

Send no request body.