Everybody needs to understand that if you do both fax and mail your requested documents you just gum up the system and make it longer for you and everybody to get their return. When you fax your information gets put in and processing begins. Then, when your mailed copy is received somebody has to physically open it and then IRS employee's time is wasted trying to process forms that are already being processed.
Just do one of the things your letter asks. If you don't trust faxing, then just mail it.
