It is possible to add text to invoices.
To add text to an invoice for a particular client:
Click Set up and then select Client Maintenance.
To enter Modify Mode, press Ctrl+M keys and click the Calc Group button.
To create a new Client Calc Group, click Copy Group.
Select the Invoice Calculation Group usually used for this client.
Type a new name for the copied version.
At the bottom, type the Invoice Narrative.
📌 Note: You have a limit of 256 characters.