Attach the prepaid return label you already hold for an OMS (Custom)
order — e.g. a USPS return label printed into the box. Get Plus never
generates or pays for this label; it is surfaced on the customer return
flow so the shopper can print/scan it (display priority: label artifact →
tracking barcode → "in your packaging" notice).

This is a returns-domain resource owned by the returns service. It is
addressed by order_number because the label is attached at fulfilment —
before any return exists — so the order is the only stable identifier at
that point. The label is not part of the POST/PUT /orders body.

This endpoint is idempotent — a second PUT replaces the stored
label. The merchant-provided label is always the first priority when a
return is created. (Get Plus-generated labels for a 2nd/3rd return may be
offered in the future; not available today.)

The # prefix on the order number is normalised, and only OMS (Custom)
orders are accepted.