Yes, you can set up standard or default message for invoices so that it is not needed to retype the same text each time for every invoice. In Global Settings > Billing, there is an option for 'Message on Invoice' that displays a message on all invoices generated from Core. You can do this at the project level also from the Projects > General > Billing Options tab. The message at the project level takes precedence.
Note: You can also control the font size and style of the message from Global Settings > Invoice Templates.