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:
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1- Navigate to the invoice by clicking on the invoices tab.

2- Select the invoice you wish to archive

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.

Once an invoice has been archived, it cannot be re-opened or paid for.