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&#x27;s a dandy reason to drive adoption of the app.<p>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&#x27;m sure it correlates with income, and gives independent verification of income, statistically.<p>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.<p>With the stores&#x27; 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&#x27; reserve price (and hence propensity for returns and chargebacks, which they surely care about), than being a &#x27;gotcha&#x27; adversary.<p>Added link: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Expected_utility_hypothesis" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Expected_utility_hypothesis</a>