The following is provided by my school. I just mailed OPT extension. 1) using U.S postal Service: USCIS Phoenix Lockbox USCIS PO Box 21281 Phoenix, AZ 85036 2)For private courier/express (non-usps) deliveries: USCIS Phoenix Lockbox USCIS Attn: AOS 1920 E. Skyharbor Circle S Suite 100 Phoenix AZ, 85034