Deleting an Invoice

In order to delete an invoice, you must first void any payments that have been applied to said invoice:

Billing -> Invoices -> Void Payments

If the payment has already been submitted to the bank you will need to delete the bank deposit first:

Billing -> Processing -> Edit Bank Deposit

Then go to the invoice edit screen:

Billing -> Invoices -> Edit Invoice

Choose the invoice and click 'edit'. Go to bottom of invoice and select 'un-post'. It can now be deleted.