Extra revenue with Google AdSense
AdSense is the facility to put a snippet of code on your site that allows Google to place adverts relevant to the keywords on your page. When your customers click on an advert, Google pays you.
There are disadvantages to AdSense as well. When a customer clicks your advert, they go to another site and this site might be a competitor's. Oops!
My view with regard to this last concern is that if your site is set up correctly and your products and categories are described with proper sales copy, then this will only happen when you haven't got what the customer wants anyway. Customers know what they want, they know your site is not their only option, and most important, they know if they like what they see or not. So why not offer them an alternative that makes you money just in case you don't have quite what they are looking for?
I have read that putting AdSense on an e-commerce site is nothing less than stupidity. My experience says otherwise. Anyway, Amazon...