Welcome to the Community, @rhudalu.
I'd be glad to guide you in adding fields in your estimate and invoices in QuickBooks Online (QBO).
Here's how:
You can also check this article for more information about adding custom fields: Add custom fields to sales forms and purchase orders in QuickBooks Online.
For future reference, you may check this article to guide you on personalizing and adding specific information to your sales form: Customise invoices, quotes, and sales receipts in QuickBooks Online.
Please feel free to drop a comment below, if you have further concerns about adding fields or other QBO-related concerns. I will answer them in any possible way.
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