To add a custom Job variable to an Invoice template, you can add the expression DTLINKJOBS: to the variable name, like this: \VAR_NAME\ => \dtlinkjobs:VAR_NAME\

Please note that this only works within the Table Scan portion of the template (i.e. between the tags \scan(dtLinkJobs)\ \scanentry\ and \scanfooter\ \endscan\)