When i applied, i never included a letter. I had photos of our marriage registration, photos of both of us with many of our extended family members, and because we are long-distance, I included screenshots of our phone conversations, 1 screen for every month since we first spoke to each other (total of 7 screens), and because I receive benefits at work, I also included a company issued form saying that once she lands, my wife will be included in my Benefits coverage.
Hope this helps you in your application. Good luck.
