Earlier quoted context omitted.
Founder here. Definitely a common first reaction, but we've found that on average shoppers become far more loyal (and spend more, growing store top-line & often net bottom line) when this happens. You might think we're crazy, but we fully believe (and have data to back up) that we're helping these stores far more than the costs.
You should take it a step further and help the retailers add this as a feature to their own apps -- having a store app should include a promise that if the store lowers prices, it will refund the difference for purchases in the last week (or whatever time frame) to loyal customers, perhaps through a store credit. It's a dandy reason to drive adoption of the app. One thing you should listen to is the number of people…
Supplying merchants is also likely to provide a more _predictable_ income.
However...
Are giants like Amazon really likely to listen to a pitch from a startup, even one incubated at YC? (No snark, genuine question.)
There is another option: open source the product and give it away, charging for the hosted service. Obviously this may not be an option for Paribus, which would be a shame.