I haven't checked the source code of the plugin, but I wonder if they add affiliate links when they link to the other prices that are lower? If they don't, I don't see why not, it seems like a nice & easy way to skim off the top of those transactions for providing a nice extension. I'll definitely try this out for a bit and gauge how good it works.
It looks like they're going to be monetizing by partnering with specific retailers. From the plugin description:
> Buy Happy retail partners will soon be giving you deals to share with friends and family via social media. And we’ll be adding more powerful features to Buy Happy as well — while building a fun community along the way. Isn’t it time to add Buy Happy to your browser?
Also, don't forget the data they are harvesting. From their privacy policy, they are monitoring your shopping habits and what you do before/after you look at a product. So, that will let them "support our relationships with the operators of the retail websites from which we provide you with information about lower priced products"[1]
It's not always easy to become an affiliate so it may take time to get a good rooster of stores. A common approach is to crawl the data even if you don't have an affiliate deal. Not only is it proactive but it also enhances the product since it will show more alternatives.
Good luck reading the code! It's compiled by Google Closure compiler, so it's highly minified. The bulk of the extension is actually written in ClojureScript.
