Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of client],
It was great speaking with you the other day, and I am excited to hear that your new interface is up and running without further issues. I am glad that we could be of service, and happy that you are satisfied with our product.
I am enclosing a copy of the project sign off documentation, which has all been completed.
As the project has been completed and signed off on, I am also enclosing an invoice for the project fees. We would appreciate if you would put this invoice in line for payment as soon as possible. As always, payment terms are thirty days net.
Please let me know if you need anything else. Thank you very much for the opportunity to do business with you.