My 2c.....
I would never ever send anything to USCIS using Fedex, DHL, Express Mail etc. Do not go overboard.....
If I were you, I would use just good-old USPS and would use First Call, registered mail. If you want, you can also add "green card return receipt" (not really necessary).
Make it simple, stupid and idiot-proof.
It is your call....