The USCIS has extended its Direct Mail lockbox program to include I-800/A forms.  Applicants were previously required to file at a USCIS field office with jurisdiction over their place of residence.  The Direct Mail program allows USCIS to process applications more efficiently by eliminating duplicate work, maximizing staff productivity, and introducing better information management tools.  Starting September 25 all I-800/A forms and supplementary documents should be sent to:
U.S. Citizenship and Immigration Services
P.O. Box 805695
Chicago, IL 60680-4118

This change does not cover I-600/A filings.