To add a new line item to your price list, go to settings - payments and invoicing - line items and select "+Add line item"
Now populate one or more fields for each line item:
In the example above, more than one location is used by the business, so pricing is set independently at each location.
If you would like to add or edit the columns for line items that show on your invoices, for example, adding a dedicated column for NAPPI codes, go to settings - payments and invoicing - line items and select "edit columns"
In the new window that pops up, you can "+Add column" or change the order of the columns by dragging the right bar.