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.