I wish it were so...that the various mailing services had that ability to verify what goes into a package before it's sealed and sent. Receiving an empty box or one filled with a few rocks (or whatever) results from a dangerous loophole in the whole transaction process. Luckily(?), it's a fairly small and relatively infrequent occurrence, because, first, you have to be a criminal (this is a pretty serious crime--oops won't cut it); and, second, you're a criminal who's pretty desperate to monkey with small potatoes profit. Even a notary public would not work, unless mr. or mrs. notary accompanied you to the post office to observe the sealing and handing over. It's a bit unsolvable at this point.
Hoppes is right, unfortunately.