You can add non-inventory line items to a sales order to charge customers for things like shipping and handling fees, or other items you're selling that aren't in your WholeCell inventory like accessories, chargers, parts, etc.
To add a line item, just click on the blue + button on the order page:
Set the description of the line item, the category, and the price, and click Save.
Now you'll see the line item has been added to the order, and it appears in the table on the order Details tab. That's it!