There are three types of 'item' that can be added to an invoice; Rooms, Extras and Payments.
Once you have created an invoice, any subsequent un-invoiced items can be added on to it using the "Link items" buttons under each corresponding section of the invoice.
Any Items that are on the "wrong" invoice can also be removed (Unlinked), and moved (Linked) to the correct invoice.
Here we have a booking with two invoices.
The second invoice was created by mistake, so let's unlink the one item currently showing on it. Click to open the invoice and click the Unlink icon. Here it's an extra, but the principle is exactly the same for Rooms and Payments too.
Click "Delete" and "Save"
Now open the first invoice, and under Extras, click "Link items". Notice the Unlink icons for the Room and Payment already on this invoice.
From the pop up, you'll have a list of all un-invoiced Items of that type. Here we have just the one.
Select it and click Save.
We now have all the items on the first invoice, and the second invoice has no items and no outstanding balance.