Another interpretation is that the person holding the package is using a roundabout way to tell the person who asked what the package was to mind his own business.
The name MacGuffin bugs me. It makes it seem like no matter how essential to the plot whatever device is, it's not needed- just an extra thing.
If it's truly important to the story, then it's not a Macguffin.
We call the Omnitrex a MacGuffin and it is the plot.
What defines a true Macguffin is the ability to completely change and have no effect on the story. Just like the uranium in "Notorious" could have been diamonds or money or documents or anything else that could fit in a wine bottle, the Omnitrix could've been a gauntlet or a magic word or a serum or anything else that would let Ben turn into different aliens.
No the Omnitrix plays a role in the plot by turning him into different aliens, you couldn't replace it with say a sack of gold. It has a 'on-screen' use while you can't use a true MacGuffin. It's the goal not the means.
Then it's been improperly labeled as a MacGuffin. The definition is quite clear here — if it matters at all what it is (besides gross characteristics like, say, weight) it's not a MacGuffin.
I don't really understand the whole "interchangable" idea, especially in the context of Indiana Jones (which is typically used as an example of what a MacGuffin is). Is the Holy Grail, from "The last Crusade" a MacGuffin? The main page says it is, whcih means it DOESN'T really do anything major, but it does effect the plot (namely healing Henry's wounds, which is something, say, a bag of Gold couldn't do.
I think a MacGuffin can be "Mostly" interchangable, I mean... in your example, it's until the very end that the Grail becomes a kind of Phlebotinum. And altough a bag of gold wouldn't make it; it could be the blood of a dragon, a magic mantle, or a wand of Cure Critical Wounds. If the movie should have more religious themes, if there are hints that both Indy and Henry are "The Chosen Ones", and the fact of the Grail being the Grail be more important that it's whole search, and it's powers and history are critical to the plot, then it's more akin to being defined as "Missing Phlebotinum".
As a better example, the Second Ewoks movie has the Big Bad Evil Guy stealing the Power Cell of Cindel's ship: Altough it's used to power the ship, it's only given any use in the very end, and in a very non-ceremonial manner (It's just plugged, just like you plug batteries in real life) It could be the power cell, the control panel, the stellar maps, the main motor, an R 2 D 2 droid, or just the brake pedal: Whatever it is, the evil guys stole it, the good guys need it, and the heroes are going to recover it.