You can delete an invoice by going to Manage » Invoices and clicking on 'Edit' from the invoice menu:
Or you can click on the invoice number, then click on 'Edit' at the top of the page. At the bottom of the edit view you will see the option to delete the invoice.
It is important to note that once you have deleted an invoice, it will be permanently deleted. This will also remove that invoice number from your sequence (ie. If you have invoices 1,2 and 3, deleting invoice 2 will show your invoicing sequence as 1, 3 and so on).
If you would like to retrieve a deleted invoice, or turn off the ability to delete invoices entirely, please contact support at email@example.com.
Should you delete your invoice or write it off instead?