Jason,
In the business that is called "being in the weeds".
Your assertion that the store needs a new Manager is based on your "snap shot - experience ".
Had you taken the time to go in and spoeak with the manager or at least called back you might have gotten to the root of the problem and not had to write this letter.
95% of problems like yours can be corrected on site if only one takes the time to share feedback with management.
One other observation. Your reference to other employees that "deserve the job over him" makes me a little suspect of the entire scenario and intent of your letter.
Bill R.