You must use the CustomizationID element to specify urn:vertexinc:vrbl:billing:1 as the ID for the Document.

Elements

Element	Description	Cardinality
`Invoice/CustomizationID`	Specify `urn:vertexinc:verbl:billing:1`. This element is an identification of the specification containing the total set of rules regarding semantic content, cardinalities and business rules to which the data contained in the instance document conforms.	1..1

Example

<Invoice>
    <!-- code omitted for clarity -->
    <cbc:CustomizationID>urn:vertexinc:vrbl:billing:1</cbc:CustomizationID>
    <!-- code omitted for clarity -->
</Invoice>