You can use <barCode> tag in rml to generate barcode.

[[ repeatIn(objects,'o')]]
<blockTable colWidths="120"  style="Table_1">
    <tr>
        <td>
            <barCode code="Code128" x="0.0cm" y="0.0cm" quiet="0" style="barcode" barWidth="1" barHeight="14">[[ o.name ]]</barCode>
        </td>
    </tr>
</blockTable>

You can study Barcode render code at Barcode Render.

If you create your reports with LibreOffice (.sxw-file) you could try something like this:

E.g.

[[ setTag('para','barCode',{'code':'Standard39','value':'o.name','barHeight':'15.0'}) or removeParentNode('para') ]]

Check the rml-reference on barCode-tag for more information:

http://www.reportlab.com/software/rml-reference/

With a barcode label generator, you can generate barcode labels for your invoice. So I still did not get essence of your problem. You have make your question more specific.