How can I send Harvard an additional letter of recommendation after being deferred?
Through the applicant portal? If yes then Harvard will know I saw the letter
The recommender can always mail it
What if I am uncomfortable asking the recommender to do that?
Will they accept a letter through the portal?
IMHO: I’d have the recommender snail mail the letter or FAX the letter to Admissions.
Playing the devil’s advocate here: Although Harvard will accept an additional letter of recommendation uploaded to the portal by the applicant, Admissions may question the authenticity of the letter. Did a teacher write the letter? Or, did a student write the letter on their own behalf? If Admissions has a question about the contents of the additional LoR uploaded to the portal by the student, they might contact the teacher to ask about the applicant. OP: If you’re not comfortable asking the recommender to mail the letter, you best be comfortable if Admissions contacts the recommender and asks about its authenticity!
I’m not sure what is uncomfortable about saying, “Here is a stamped envelope in which to put your rec.” vs. “Give me your rec so I can read it before uploading.” Additionally, Harvard may question the value of a LoR that was uploaded vs. mailed. YMMV.