IMPORTANT: This is a full-replace operation (PUT semantics). All fields must be
included in the request body — omitting any field will set it to null, wiping
existing data. Always call retrieve-an-invoice first to fetch the current record,
then include ALL existing fields in your update request along with your changes.
Metadata is the exception: omit metadata to preserve all existing metadata, or
supply only the metadata keys you want to set or clear.