One of the difficult things about shipping into the US is that the USPS does not give incoming international registered mail, registered treatment. Registered mail is supposed to receive special security treatment by the post office and is also supposed to be signed for, but for some reason, the US does not give this treatment to registered mail coming in from overseas.
Hence, if you are tracking the item, the tracking effectively stops since the US will not reliably scan the item. Nor is the item treated with security as registered mail is. There is essentially then no difference between how a registered item is treated as compared to one that is ordinary first class mail.
If I were shipping an item of value into the US, I would only use one of the major private couriers, Fedex or UPS, specify signing.
Not sure if this helps but if you've sent the item in as I think you did, there is really no way to tell or prove what has occurred once the item crosses into the US.