Only User accounts with permissions of Super are able to delete Invoices. Do be careful though; deleting an invoice is final and there's no going back or recovering it later on!
If you're really sure you want to delete an invoice, follow these steps...
1. Go into your invoice
2. Select Options (top right) and Delete
3. The page will jump to the base with a warning, an opportunity to print the document, and then final confirmation. Check the box to confirm, and hit delete
...and hey presto, your invoice has been deleted!