I sent an e-mail to the USCIS in regards to what service we can use, and this was their response:
Good Morning:
The method of shipping is your choice. Please note the difference in mailing address if you choose to use the USPS rather than another private courier. Private couriers do not ship to PO Box addresses.
The Lockbox Support Team
U.S. Citizenship and Immigration Services
From: Cookie
Sent: Friday, August 17, 2012 4:35 PM
To:
[email protected]
Subject: What type of Courier Services Can I use
To ship to any of the overnight lockboxes can I use FedEx, UPS or any other carrier service or my only choices is the United States Postal Services(USPS).
Thank you in advance for your time and attention in this matter.
FYI: I used USPS overnight and got a signature copy when I used the service.