You can quickly add an internal or external comment to an item by clicking on the pencil icon at the end of an item's row.
Then select from the drop-down if it's an External Comment or an Internal Comment. Then enter your text and click Save.
It'll be present when you export it via the Custom Export.
Group Edit Comments
If you want to add comments to multiple line items at one time, you can do so using the Group Edit drop-down menu. The Set Comment is the option there, which you can use to set and group edit the internal/external comments for multiple lines.
Select in your list the line items that you want to add comments to and then from the Group Edit drop-down, click on Set Comment.
Removing a Comment
In the expanded view, you can remove the external comment or internal comment by clicking the trashcan icon.
It's also possible to remove both Internal and External Comments from the Comments modal. Simply delete your comment and click Save to delete your comments: