> Right, which drives up transaction volume which in turn means the payment network and the seller get more volume and more money.

It only drives up transaction volume for high risk transactions, which is bad because those are the ones that increase the amount of fraud.

> The risk is accounted for in interchange.

The risk is magnified from fraud taking advantage of the insurance. You have to pay more because the insurance is a deep pocket to steal from with weak ability to protect itself because the actual parties are no longer vested in preventing it. Or the cost of the "insurance payout" is paid by the merchant who knows that they're innocent but the payment processor has no way to know that.

> The bigger your risk pool the lower your cost. That's how insurance works. The more people you can socialize big losses over, the less you have to charge each person.

That's not how insurance works at all. You buy insurance because you and some other people each have a 0.1% risk of losing $100,000 and you would each rather pay $100 with 100% probability than $100,000 with 0.1% probability. For this to work it makes no difference whether there are a million other people or a billion. It only matters if you get down to something like 1000 people and even then your expected value is still the same, but there is a greater chance that your premium ends up as either $200 or $0 instead of $100. But we're not talking about population numbers that low here anyway.

And the socialized losses are the issue, because that's a moral hazard. Once you're insured you take bad risks and rely on the insurance to eat it, which raises costs for everybody.

When you have to insure a $100K+ loss you can't afford to suffer, you're willing to eat that overhead. But to insure a $100 loss? It makes no sense. You're going to make enough $100 purchases yourself that you can "self insure" cheaper than an insurance company with bureaucratic overhead and moral hazard can do it for you. It costs less to lose your modest purchase price a small percentage of the time than to pay a higher than that percentage for the insurance. And the amount you lose buying the insurance is inherently more than the amount you lose of your own on average unless the insurance company is losing money.

> Your margin on top is a function of your business goals. It's not strictly true that more people equals more money, you can always pass on the costs. Depends on where you make your money. A 501(c)(3) that offers insurance for instance would just charge less.

If your net margins per transaction as an insurer are a given percentage, you make more money by processing more transactions unless that percentage is zero or negative. A non-profit may purposely have zero margins, but then in what sense is it "in their interest" to have greater volume? (And how many non-profit major banks are you aware of?)

> 3% for card present is actually really high, I was using a blended average of the ~2.5ish% charged for card-present, 3ish% charged for online and 3.5ish% charged for card-not-present transactions at the point of sale to small and medium sized businesses. You can expect this to be 1% lower for merchants of substantial scale.

You're implying that "merchants of substantial scale" are paying an average of around 2%, or around 1.5% for card present transactions, for cards paying 2% cash back. That seems a bit fishy, doesn't it?

Square is using "let us mush all this together and average it out for you" pricing, but that only really works if they reject merchants with high chargeback rates etc., and still requires them to raise their rates if cards that give higher rewards by charging higher fees become more popular.

Meanwhile most other payment processors are going to impose chargeback fees on top of that, which is what we're really trying to avoid here -- plus the major cost that isn't in any of these numbers which is the cost of lost merchandise and labor when there is a fraudulent chargeback for goods and services already rendered.

> Again, see [2].

Right, so the merchant is paying 2.9% to Square, but the poor customer has a 0% cash back card because with their credit history it's all they can do to get a credit card offer at all. Then too many of the merchant's other low income customers commit fraud and they get kicked off of Square and have to suffer the high end of "between 2.87 percent and 4.35 percent" from whichever of Square's competitors will take them.

This customer might be quite pleased to precommit to not doing a chargeback with a trustworthy merchant in exchange for a ~4% discount.