To add a product start by using the side menu and clicking Once-Off Invoices:

You will then be taken to the once-off invoices page:

Here you will select which client you are sending the invoice to, the invoice’s due date, then choose which product is being sold and the quantity being sold, then select whether VAT is included or not, and then click save. You will then be taken to this page:

Here you can either Click the blue edit button to the right to make changes to the invoice, or you can export the invoice to a PDF document or send it immediately to the client.