If you've submitted an invoice or invoices to a client and still haven't received payment, it's a good idea to send a final invoice. This lets the client know that this will be your final demand for payment before you enforce your rights to compensation -- including possibly filing a lawsuit to collect payment.
Nolo's Final Invoice eForm lays out all the necessary components of a final invoice so that you can get paid. Plus, you'll get clear instructions in plain English to help you fill in the details and guidance on which member of your organization should sign the form.