Why in the world would you pay for something like this when there's the insanely awesome YOURLS? It's free, written by two well known WordPress plugin developers (Ozh and GamerZ), and doesn't even require WordPress (although of course there's a WP plugin to auto-shorten and even auto-tweet new posts or pages).

You can run it on your existing domain (in a subfolder or whatnot) or buy a cheap short domain. I'm paying $10/yr for v007.me. :)

Here's a screenshot of the WordPress plugin at work on the write screen, and also an example of the thing in work (I used a custom stub, but if left it at the currently single character default, my URLs are currently shorter than bit.ly URLs and will be for quite a few hundred or thousand posts): http://v007.me/yourlsimg