If it's not too much hassle it may also be worth it to send the paper letter via 2-3 day mail (in a one of those big cardboard envelopes).
This is the mail equivalent of the restroom key chained to a giant log - it makes the letter harder to ignore or misplace (as a bonus you can get delivery confirmation - which is a great entree to a phone call - "my letter was delivered on XYZ date ...".).
You want to use certified mail, return receipt requested.
Then you get the little green card with a signature or a signature stamp.
