By default, item options (like color, size, or other customizations) sent from a selling channel are listed horizontally. However, you can make a small change to your packing slip template so these items appear as a vertical list instead.
To update your template so item options appear as a list, go to the Packing Slip Template in your template settings and do the following:
Click the Order Items header in your editor.
Add "Without Replacement" to the [Item Options] field replacement (do not include the quotation marks).
Your item options will now print in a vertical list, similar to this image: