What is not working to add an "Special" line item to an order/invoice?

API Q&A
,
1

Below is the request body to /xmlrpc/v1 endpoint(POST).
I made sure private key is correct and full, copied from another successful request.
I’m using Make.com to make this call, and cannot use any libraries.

<?xml version='1.0' encoding='UTF-8'?>
<methodCall>
  <methodName>InvoiceService.addOrderItem</methodName>
  <params>
    <param>
      <value><string>PRIVATE_KEY</string></value>
    </param>
    <param>
      <value><int>37514</int></value>
    </param>
    <param>
      <value><int>0</int></value>
    </param>
    <param>
      <value><int>7</int></value>
    </param>
    <param>
      <value><double>-210.00</double></value>
    </param>
    <param>
      <value><int>1</int></value>
    </param>
    <param>
      <value><string>Price Adjustment from Shopify Sync</string></value>
    </param>
    <param>
      <value><string>Review tax calculation.</string></value>
    </param>
  </params>
</methodCall>