[Your letterhead, if desired; if not, your return address]
[Date of letter-month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I hereby authorize you to perform plumbing repairs in my home at 1239 Plumb Avenue, as previously discussed. The repair date that we agreed upon is August 17. I look forward to seeing you then, and please let me know if you have questions in the meantime.
[Sender's first and last names]