You can't customize the simplified packing list from the create label page, but you can customize the detailed packing slip accessed from the drop down menu on the label management page, including adding your logo.
Another more eye catching way to do this, since the packing slip is PDF, is when it opens in Adobe, click on the Comment option, then either mark a starting place or draw a text box and type or paste your message. Doing it this way allows you to add a big bold, even color or special font message that will be much more noticeable.
