Hi there, userinfo110.

I'll help with your question about managing invoices. 

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:

1. Go to the Sales menu and proceed to the Invoices tab.
2. Look for the invoice and click on it.
3. Select Edit invoice in the lower-right hand corner.
4. Add the products or service items from the second invoice.
5. Click Save and send.

Then, follow the steps in this article on how to delete a transaction: Void or Delete Transactions in QuickBooks Online.

You'll also want to take a look at this article in case you want to add customization to your sales form: Customize Invoices, Estimates, and Sales Receipts in QuickBooks Online.

The Community is always here if you need anything else. I'd be glad to help you again.