If you'd like to delete an invoice forever, here's how:

1. Click on the Menu icon and go to the Invoices page.
2. Find the invoice you'd like to delete, click on the 3 vertical dots to open the menu and click on Delete.

Alternatively,

1. Click on the Menu icon and go to the Clients page.
2. Search for the client, click on them and click on the Invoices tab.
3. Find the invoice you'd like to delete, click on the 3 vertical dots to open the menu and click on Delete.