You should have received it by now. It probably got lost in transit.
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How do you know it was approved if the online case status tells you initial review and you did not receive a green card or welcome letter in the mail? If you know for sure it was approved, you should contact USCIS directly for a replacement document, probably via info pass.
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It is unclear how you know your I-485 is approved if you have not yet received a decision. A decision is typically issued 2-4 weeks after the interview bit can be delayed due to security/background checks, criminal record, etc.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter. No recipients of content from this answer, clients or otherwise, should act or refrain from acting on the basis of any content included in the answer without seeking the appropriate legal or other professional advice on the particular facts and circumstances at issue from a licensed attorney. Provision of information on this website does not create an attorney-client relationship between you and The Law Offices of Grinberg & Segal, P.L.L.C., nor is it intended to do so.
Check the web site, since it likely got lost in the mail. You can also request a new copy.
Most lawyers will give you a free consultation if you ask for one. We do this all the time, but remember that our time is valuable and you should stay with the people who help you the most.