Does USCIS office accept USPS Express Mail with return receipt?

I sent out my I-751 (petetition to remove green card condition) to USCIS California Service Center (PO Box 10751, Laguna Niguel CA 92607-0751) yesterday via USPS's Express Mail with the Return Receipt option. I wasn't sure at first whether the PO Box address will accept signture return receipt but the post office lady said they should. Anyway, I just track online at USPS site and it said:


Status: Notice Left

We attempted to deliver your item at 11:10 AM on April 3, 2007 in LAGUNA NIGUEL, CA 92607 and a notice was left. A second delivery attempt will be made. If unsuccessful, we will hold it for five business days and then it will be returned to the sender. Information, if available, is updated every evening. Please check again later.

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I am not sure if I should do the return receipt and what if they don't accept it for second delivery? The package will return to me? :(

Please share your experience and what kind of mailing option I should use? Thanks!!