Returns the order's payment timeline, oldest first. Each billing event is one row: a scheduled payment or a standalone payment.

• Refunds, captures, reversals and chargebacks don't get their own row. They sit in related_transactions under the charge they belong to.
• To act on a row, use its IDs. For a scheduled payment, send spid to the scheduled payment endpoints. To refund, capture or reverse, send the transaction's id to the Payments endpoints.
• A draft order returns an empty list.