curl --request POST \
  --url https://api.arcuserp.com/v1/orders/{id}/items \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "product_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "quantity": 2,
  "variant_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "sell_rate": 123,
  "list_price": 123,
  "notes": "<string>"
}
'

{
  "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "order_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "product_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "variant_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "sku": "<string>",
  "title": "<string>",
  "quantity": 123,
  "quantity_fulfilled": 123,
  "quantity_returned": 123,
  "sell_rate": 123,
  "list_price": 123,
  "discount_amount": 123,
  "line_subtotal": 123,
  "tax_amount": 123,
  "line_total": 123,
  "unit_cost": 123,
  "weight": 123,
  "is_tax_exempt": true,
  "product_tax_code": "<string>",
  "sort_order": 123,
  "created_at": "2023-11-07T05:31:56Z",
  "updated_at": "2023-11-07T05:31:56Z"
}