I have used this a couple times as sender and recipient and it is very successful. Even though the post office pretends everything is in transit, it appears they know precisely where to find our parcels in some warehouse. They are non-committal about the results (and then they ask right away for a review of their services*), but I find the packages get deliver in a day or two after my inquiry.

My packages always have tracking. All bets are off for regular letters.

*how can I review anything about the post office before I get results - i.e., delivery?