I contacted the Quickbooks helpdesk today to find out how I could add a PO number to an invoice, so my client would be able to locate the contract and pay me. I was very disappointed to find out that Quickbooks Online Simple does not offer an option of creating one extra field on an invoice - I was told to upgrade if I want to be able to add a number. I will either have to download a pdf and add a PO number manually or stick with Wave which enables one to do it - for free!