Jeez!....
$89.99 for a little mirror!!!..
Save your money mate.
Go and buy a builder's laser from your local builders store..
I got one for $30.
Make sure it can create a vertical line, rather than just a dot.
Put a line on your putter from the sweet spot to the back center of the putter (use thin coach stripe you can buy for cars) and use that as your alignment mark with the laser line....
I use a mallet putter, so there is plenty of room for a nice long alignment line on it...
Here's a pic. it's the RED line you want:
http://i121.photobucket.com/albums/o233/andrew7_bucket/image002.jpg
The laser line will also help you get your eyes directly over the ball, otherwise the line will look bent as you look down on it. When it looks straight you are directly above it.
You can watch the putt run down the laser line as well..