Creates a new order or updates an existing one (upsert). If orderId is omitted from the URL, one is auto-generated.

The order type is determined by the data provided:

• Include a paySchedule object with an amount to create a Payment Plan (finite, ends when paid in full).
• Include a paySchedule object without an amount to create a Subscription (recurring indefinitely until cancelled).
• Omit paySchedule entirely to create an Unscheduled order (simple one-time invoice).

When providing pay schedule data for the first time, paySchedule.frequency and paySchedule.recurringAmount are required. When updating an existing pay schedule, both fields are optional (existing values are preserved). The reminderBeforeDueDays and retryAfterDueDays arrays must contain positive integers that do not exceed the frequency's maximum offset days (daily: 0, weekly: 6, monthly: 27, yearly: 364).