I haven't had a chance to check out Buy Happy yet, but it seems to me that displaying competing prices on product pages rather than when adding to cart might be a better experience. I use price as one of the deciding factors on whether to buy, and there have been a few times where the lower price that Invisible Hand found convinced me to actually buy a product.
Perhaps both approaches are useful.
The reason the cart method is powerful is because shipping can be compared (FS on $X+) as well as promo codes or other cart-based deals.
Not trying to be trollish, but I genuinely dislike this sort of monetizing and wish these extensions were at least more upfront about it.
Do you have a reason for feeling this way, aside from not wanting someone else to earn money from your action?
They are providing you with a service that saves you money, so I don't understand the hesitation.
It's the third item on their FAQ (http://www.getinvisiblehand.com/faq). That's hardly hidden. I don't even use these services, but it seems like a reasonable way to make money.