Simply replace the unfilled areas such as "" and "[date]" with your information.
Re: Invoice for [Project Name] Consulting Services
This letter is to serve as an invoice for the [Consulting Services] consulting services provided to you for the [Project Name] project. Work for this project began [start date] and was completed on [end date]. Per our contract, all deliverables have been met. As agreed upon, the project services and deliverables provided were:
The invoice total is [total]. This is payable within [payment period] days of this invoice date. All supporting documents and receipts are enclosed.
I appreciate your business and look forward to working with you again. Please call me if I can be of service to you.