Hi jcosentino.
You're in luck! There is a way to include a brief message in your invoices.
Simply click on the drop-down arrow under the Customer Message section, then select a message or click on Add New to create a new message. In your case "We're all in this together"
Once you finish this, your changes will be displayed when printing the invoice.
The following link breaks down the process further. It also includes step-by-step pictures, which are helpful for anyone who needs visual aids.
If you need further assistance, I'm just a few clicks away. Hope you're having a great Wednesday.