They send the letter right after your approved, but I never received mine. Fortunately when I was on the phone with IMMG the officer brought it up and said i’ve been approved!
Told them I never received the letter, they said to wait, I should within 15 days. I still didn’t so I called back to tell them I still haven’t received it. So they had to send another request to forward the letter, I still haven’t received it but fortunately I got an email copy of the official letter after the second request.