Can I insert a line within an invoice

Let me make it up to you by ensuring you can add the date of service on the invoice in QuickBooks Online (QBO). 

QuickBooks lets you track and edit invoices. We can open the invoice and add the date of service. 

Here's how:

1. Go to the Sales menu and click the Invoices tab.
2. Locate the Invoice.
3. Click the drop-down arrow under Action and select Edit.
4. Make the necessary changes.
5. Click Save and close.

I've added this article for more information about recording payments you've received from your customers: Record invoice payments in QuickBooks Online.

You might want to check this article to know more about customizing sales templates in QBO: How to customize invoices, estimates, and sales receipts.

Let me know if you have questions about editing invoices. I'm always here to assist. Have a wonderful day!