Can I add a new product to the database after I've already started ringing up an invoice?

If you are in the middle of ringing up an invoice and you realize that you need to add a new product, you can click the + icon next to the search products field. This will display the fields for you to enter the product's information right there in the invoice. 

If you are in a hurry, you can go back and add more details later. To do that you would access the product from the product list screen.