If you wish to add a barcode to a Channel Advisor invoice or a Packing Slip, Channel Advisor advise below:
To add a barcode you use the template tag {{BARCODE(tagname)}} where tagname is the tag you want to convert.
Further information is available on the Channel Advisor website:
Customise Packing Slip Template
If you have any questions relating to Channel Advisor, you can contact Channel Advisor Support