You can add internal notes/comments to invoices within MIDAS, which will only be visible to MIDAS users who are able to view the invoice. Internal notes do not appear on invoices themselves, and so are not visible to the client a given invoice is for.
To add/update internal notes for an invoice:
Select the icon in the main toolbar.
Locate and view the invoice you wish to add/update internal notes for.
When viewing the invoice click the "Internal Invoice Notes" icon from the Invoicing toolbar shown above the invoice.
An "Internal Notes" editor will then be shown, allowing you to add/update internal notes currently associated with the selected invoice.
Once finished, click the "Internal Invoice Notes" icon from the Invoicing toolbar again to hide the editor and save the internal invoice notes.
Tip: If you've enabled online payments for your invoices, then when your client pays it will also be noted in the Internal Notes for their invoice (in addition to being recorded in the Invoice History log)