I'm here to give details about customizing invoice temple in QuickBooks Online(QBO).
Currently, the option to add columns isn't available. Instead, you can add custom fields for your invoice. I'll show you how.
For your reference, you can review this resource for more info: How to add custom fields to sales forms and purchase orders.
In addition, you can check out this material to learn more about customizing your invoice template: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Feel welcome to visit the Community if you have any other concerns. We're always here to help you.