Another improvement to our accounting software based on feedback from our customers:
In KashFlow you can add private notes to an invoice. These are shown at the bottom of the invoice page and are NOT included on the invoice itself when it’s sent to the customer (unlike "comments" which you can add to an invoice by setting the Qty and Rate to 0)
There is now an InvoiceNote class in the accounting API.
You can get all notes on an invoice using GetInvoiceNotes and add a new one by using InsertInvoiceNote.