Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of client],
Enclosed please find a copy of all documentation regarding your annual income taxes for this year. Please note that your taxes have been completed and submitted, and all tax obligations have been satisfied. It is always a good idea to keep all the attached information on file for future reference.
In addition to copies of your annual income taxes, please find an invoice for our services. Please note that payment is due within fifteen days of receipt of invoice.
We would appreciate if you put this invoice in line for payment. We have enclosed a self-addressed payment envelope for your convenience.