<p>How would I submit an additional LOR from my research mentor? It says on the Princeton admissions FAQ that the letter is welcome, but doesn't explain how to send it.</p>
<p>Mail or email.</p>
<p>Be absolutely sure that the recommendation from your research mentor adds a dimension that your application is otherwise lacking. If its contents prove to be not to different from your three required rec letters, it may dilute their effect. Most students do not submit an additional letter.</p>
<p>If you think the letter will provide an extra dimension, you can proceed with cautious optimism. :)</p>