Save your money. I've seen this thousands of times. It appears to work. All the charter boat docks use it around here and if any place draws flies it's the cleaning tables at those docks. /forums/images/graemlins/crazy.gif
All you have to do is take a heavy duty clear plastic bag (Ziploc, Glad, Hefty or whatever you like) fill it with water, tie a string around it, hang it close to where you're going to be.
Charging 7.95 for a plastic bag seems a bit of a stretch. /forums/images/graemlins/shocked.gif