The XML wrapper structure for all SOAP messages.
Every SOAP message is an XML document with this structure:
1. Envelope (root element) — declares SOAP and service namespaces
2. Header (optional) — carries metadata: auth tokens, transaction IDs, WS-Security
3. Body (required) — contains the operation request or response
SOAP 1.1 uses namespace http://schemas.xmlsoap.org/soap/envelope/
SOAP 1.2 uses namespace http://www.w3.org/2003/05/soap-envelope
The Body contains exactly one child element: the operation (e.g., CreateShipment) using the carrier's namespace.