While this is cool, isn't the concern that a retailer would get upset and change a policy to prevent this from happening? What's the end game here?
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.
One thing you should listen to is the number of people who want a reserve price (transaction threshold). This price setting is pure economic gold as information goes -- it tells you their risk aversion (probably) and the shape of their utility curve for extra income. I'm sure it correlates with income, and gives independent verification of income, statistically.
If you have 5 big retailers using your app feature internally, offer to build them a network they can join, to make their rebate offer even more attractive to participants.
With the stores' aid, you can circumvent the problems with email permissions, which frankly are going to limit your adoption. You are far better off working with the stores in a partnership I think, and helping them to understand their own customers' reserve price (and hence propensity for returns and chargebacks, which they surely care about), than being a 'gotcha' adversary.
Added link: https://en.wikipedia.org/wiki/Expected_utility_hypothesis
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.
Disclaimer: I work for Refund Retriever, which has a similar business model.