I've no idea. If you used USPS Priority Express, it should have been a guaranteed delivery. However, since it's Christmas time, anything goes with USPS. I really can't say. I've known of cases where USPS didn't update that a package has already been delivered. Didn't you see, "Please note, Priority Mail Express postage refund eligibility is adjusted for shipments mailed between Dec. 22 and Dec. 25." at the PO. Because they know there could be screw-ups.

Even if USCIS had received your package today, there's no guaranteed that they would have processed it today itself because it really depends on how fast they work. They process it by the order they received the packages.

As for whether they'll reject your package, I don't know. 😨 Just wait and see...