To remove an Invoice that may have been used for testing purposes or for any other reason that is marked as paid, simply follow these steps:
1- Navigate to the invoice by clicking on the invoices tab.
2- Select the invoice you wish to archive (you will notice the archive button isn't visible).
3- Click on the payment(s), and when its dialog opens, select "Delete".
4- Once all payments are deleted from the invoice, the "Archive" button will appear at the bottom right of the invoice dialog.