I can guide you on adding a memo to your invoice. There are different ways to achieve this in QuickBooks Online (QBO). You can either add a memo field on the invoice from your Account and Settings or customize your invoice template.
To add custom fields, please follow the steps below:
Go to the Gear icon.
Under Your Company, choose Account and Settings.
Select Sales, then go to Sales form content.
Enter Memo on the dialogue box beneath Custom fields.