I would like to add to an existing invoice

I've got steps to ensure you can edit an existing invoice, @gin1-wingfield.

You'll just have to open the said invoice, and then perform the needed modifications from there. Here's how:

1. Go to the Sales menu at the left pane.
2. Choose the Customers tab, and then look for the customer name associated with the invoice.
   
   
3. Open the said invoice, and then make the need changes.
4. Click Save and Close when you're ready.
   
   

If the invoice was already sent to your customer, you can simply add notes about the changes made on the Message on invoice box. Then, resend it once ready.

You'll want to personalize your invoices to get the info and formats you want. For more details, please check out this article: Customize invoices and other sales forms in QuickBooks Online.

Also, you're able to email or print your invoices in bulk to make your task easy and efficient. For more insights on how to achieve this, please refer to this article: Email or print multiple invoices and sales forms.

Lastly, you may want to pull up some invoices-related reports to keep track of them. Just go to the Reports menu, and then explore the Who owes you section under the Standard tab. Please see the screenshot below for visual guide.

Feel free to comment back if you need more help regarding this or any QuickBooks concerns. I'll be here to help. Have a good one.