Use this endpoint for platform: "custom" orders. It creates the
order on the first call and replaces it on later calls, and is the
recommended way to send custom-platform orders.

• 201 on first create — data.newly_created: true,
  data.applied: true, plus protect_order_number and
  order_number.
• 200 with data.applied: true and data.fields_changed_count
  when the request body differs from the stored order.
• 200 with data.applied: false and data.reason: "no_change"
  when the request body matches the stored order.

The path identifies the order

The path parameter order_number identifies the order. If the
body also supplies order_number, it must match the path; otherwise
the request fails with 422 PATH_BODY_MISMATCH. You may omit
order_number from the body — the path value populates it.

Fields that cannot change after the first create

These fields are set on the first create and cannot be changed by a
later request: platform, order_number, order_id,
protect_order_number, is_protected, order_date_created,
platform_domain, source_channel, api_version, and
order_lifecycle_status. On later requests, send the same values or
omit them — both are accepted.

Server-injected fields

store_id, platform_domain, currency, source_channel, and
api_version are server-injected from the JWT-derived Store
record. Client values for these fields are silently dropped.

Line-item identity across updates

line_items is replaced wholesale on every applied change, so the body
MUST contain the full, current set of lines. Get Plus owns a read-only
line_id per line — the stable key returns are filed against (the
line_id used by POST /returns).

• On the first create you MAY supply your own line_id on each line.
  It's honoured only when every line carries a non-empty, order-unique
  line_id (then line_id_source: partner); otherwise Get Plus mints
  li_<hex> ids for all lines (line_id_source: protectplus).
• On every later update Get Plus carries the id forward: a line
  that still matches an existing line (by product_id/sku) keeps its
  line_id; only new lines get a new id. A line_id sent on an update
  is ignored — the id is fixed at create. Removing a line, or
  swapping the product/sku on a line, drops/replaces that line's id —
  so prefer quantity edits over re-keying a line you may still receive
  returns for.

Guard: if an update would remove or re-key a line that already has
a return request, the request is rejected with 409
(code: LINE_HAS_ACTIVE_RETURN) and the offending lines are listed
under line_items. Adjust quantities instead, or leave the line in
place. (Lines without returns can be removed/changed freely.)