I notice there are no invoice numbers. Can I add an invoice number?

Yes, there actually are invoice numbers, which are optional. If you do use invoice numbers, the system will automatically number them and remember which invoice you last created. For example, if you create 5 invoices today, the system will number them 1 through 5. Then next week, you create 5 more invoices, the system will then start those with invoice number 6 and number them 6 -10.