Just add the note in a text box in the template of the sales invoice. That way the note will automatically appear on all invoices.
ie Start to print the invoice from the "print/email invoices" icon on the main screen. Click on the icon, then choose "advanced filters" then next to the words "select form for sale" click inside the drop down box and choose the invoice form/template you usually use. Eg "tax inclusive plain paper invoice" or whichever one you usually use.
Then click "customize" in the bottom left hand corner.
Scroll to the bottom of the page. You can easily add a text box here and add the note you want.
Don't alter the pre-printed boxes as they get info from card files.
Before you exit, do NOT save the form as you want to keep the original intact (you might hate your changes or you might have messed it up a bit, accidentally). Instead, click on the drop down arrow on the top left corner (just BELOW the save and print icons) and "Save As" and give it a new name that you'll recognize.
You can then see if it works with your invoices ok. I.e. before printing or emailing, select this form and print and check the box looks ok.
Back to invoices. When you're actually emailing the invoiice: from inside the invoice, at the bottom of the screen it offers "Select from" click on it and choose the new template you've made. Do this when emailing and when printing invoices.
End of part one!!
***Optional/Advanced:***
Part two
When you get sick of having to choose this each time and if you're really confident, then you can go back and edit the default one you usually use. Or: better voice?:
If you use a different template/form for different customers, you can choose which one is appropriate by choosing it in the client card file.
You can do this while in an invoice. Click on the arrow on the r.h. side of the customer name (in the invoice) to open the Customer card.
Go to the "selling details" tab at the top (Profile, Card details, Selling details)
You'll see Sales layout", "printed form". If "printed form" is available, choose the new form (template) you've made and then tap on "ok" and voila! Invoices for this client will email to or print using that form/template you made. No more extra steps. It'll be automatic. You might need to save the invoice and then re-open it for the new default to take effect. But all new invoices for that client will be based on your new chosen template. [cue: applause (I hope.]
I hope this is helpful for you. It's not as intimidating as it might sound. Give it a go!!