Quick notes can be used to leave comments on a receipt in a hurry. Simply press the % icon or name of the product you wish to leave a comment on, click on the note you're looking for, and Save.
To enable quick notes, go to Settings -> Configurations -> Shopping Cart -> Quick Notes.
You can configure the amount and content of the quick notes in the same settings.
This is how the note will show up on a receipt (the quick note in this example is “Exchanged product”):
