Adjustment of Status - I-485

So Uscis mailed me my card from MO on Oct 31st to Houston tx. On nov 1st usps tracking number changed to card was delivered to your mail box. I checked my mail box and there was no card in there. I called uscis and filed a complaint, I was told to have the post office issue me a letter stating it was an error on their part, however the post office has refused to accept any responsibility and insist the card was delivered. I even told them that the timeline for the package seems a bit because ive never received a package from us is after it was shipped the next day, it usually takes about 4 days to arrive. What do I do next because my employer needs my card to continue employment. Thanks