We're unable to merge invoices or any other types of transactions in QuickBooks Online. You can edit the first invoice and add the details from the second one. Then, delete the second invoice so it will not become a duplicate.
Here's how:
Go to the Sales menu and proceed to the Invoices tab.
Look for the invoice and click on it.
Select Edit invoice in the lower-right hand corner.
Add the products or service items from the second invoice.
Click Save and send.
Thanks
