Thank you for choosing QuickBooks as your business partner, Griffo123.
Let me share an idea about invoices in QuickBooks Online.
Each version of QuickBooks Online has different features and functionalities. And for now, adding a custom field is only available in the Essentials and Plus version.
You can also utilise the message field to add more details to your invoices.
Also, you can check this article to learn how to customise sales forms QuickBooks Online.
Click the reply button below for questions and clarifications about invoices. I'm always here to help. Enjoy your week!