You can invoice a percentage of any of existing Line Item within an Invoice. This is especially handy when invoicing part payments.
To invoice a percentage, Press the Icon whilst holding down the Option key (Mac) / Alt key (Windows) option/alt ⌥.
Enter in the percentage that you would like to invoice and select the Ok button.
If you have multiple items to add with the same percentage needing to be invoiced, hold the shift key in combination with option/alt ⌥.