Paribus (YC S15) saves you money when items you purchased online drop in price

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

I strongly disagree with this, and I have some real world evidence to backup my point of view that companies want customers to take advantage of these policies. Walmart has a feature in their app called Savings Catcher. When you buy something from there, you simply hold the receipt's bar code under your phone's camera for a few seconds. They automatically keep track of the prices of every single item you bought. When prices drop within the policy period, it is added to your Savings Catcher balance, which can be transferred to a virtual Walmart gift card for spending at any time the customer chooses.

If one of the world's largest retailers finds value in automating this process for their own customers, I can't imagine that other companies with this exact policy in place have a problem with people taking advantage of it.

(in case my comment was unclear, I'm talking about companies blocking refund services like this)

It's pretty easy to add a rule in most email providers that forward only message matching certain patterns - you could even provide documentation of what patterns to include.