The following info from:
http://www.immigration.com/fromtheagency/nsc112006.html
Cases filed by overnight courier
On the other hand for cases that has to be filed by overnight courier that is by the USPS Express Mail the center will only pick up mails from the U.S Postal Service once a day and in the morning. Therefore any mail that is not picked up in the morning will be picked only the following day and is given a next day receipt date.
Note: The package will be delivered to the Service Center P.O.Box even if the mailer is addressed to include the street address for the Service Enter with or without the P.O Box and this confirmed by the U.S Postal Service.
Filings other than USPS, like the FedEx or UPS can be addressed to the following address:
Nebraska Service Center
850 "S" Street, P.O. Box 87485
Lincoln, NE 68501-7485