Deferred Stanford. Sending Letter of Continued Interest

What are some key points to hit in this letter? And when should I send it? I’m definitely going to send the Optional Update Form around Feb 10, so should I send this letter earlier?

By mail or email? I read the CC article where it says mail means it’s directly added to your application, but I also don’t want it to get lost along the way. Or send the same letter both as an email to undergrad admissions and physical mail?