Just a reminder to those building PPC campaigns using Google Adwords that ecommerce is much different than lead generation.
For starters - always make sure that you bid almost as close to the minimum as possible. Check it daily - then add .05 to .10 to the minimum bids until you get the desired CPA. THIS IS A MUST!
Here is also a good slideshare offering more tips.
http://www.slideshare.net/Promodo_en/winning-27241411