I'll reiterate what I posted as a comment in the MacNN article regarding this:
These aren't widgets in the strictest sense.
Basically, they've created a bunch of image files for the iPod's screen, with the tips precalculated for dollar amounts between $1 and $500. Presumably, you access it like any photo album on your iPod. In total it looks like it takes up 10 MB on your iPod.
Personally, I'd rather spend an hour making my own version of this, optimized for my iPod's screen, rather than shelling out $5.
That's just me though, I'm sure this will be useful for many users.
Just don't think you're actually getting a widget programmed for your iPod. It's a bunch of image files, is all.