I should clarify one thing: because debit interchange is primarily a fixed fee ($0.21), Square's percentage-only rate actually hurts them on debit transactions

I should clarify one thing: because debit interchange is primarily a fixed fee ($0.21), Square's percentage-only rate actually hurts them on debit transactions below $10. They lose money. Of course there's a correspondingly large margin on higher-value debit transactions for the same reason. Also worth noting that Square Cash uses debit, so they're actually losing >$0.21 every time you send money with it. Stripe definitely has a large gross margin in all cases. Even AmEx. However, one thing to bear in mind with both these guys is that the easier they make it to sign up and process payments with them, the easier it is for criminals to run fraudulent transactions through them, which they ultimately eat.