[olliej]

the vast majority of that 3% is the CC company rate, and that's not going to stripe.

Which means that stripe is making 30c + maybe some tiny additional amount per transaction.

Their hardware PoS devices cost money and/or rent, so they make a profit there.

The distribution cost for their app is covered by amazon.

Which means that their remaining per transaction cost is the cost of the payment details which is presumably in the order of 1kb, which I'm fairly sure is in the fraction of cents realm.