Merchants pay what they’re charged on a transaction by transaction basis. That is unless they have some agreement with Stripe or similar that provides a fixed percentage.

If you get a full merchant account as a business, with IC+ billing or similar, your statement will itemize IC charges on every transaction and it’s based on the type of card the customer used.

If it’s Amex / Discover it might be 5% or more. This is why a lot of merchants are Visa and Mastercard only. If your customer has a cash back card, it will maybe be 4%. If your customer uses a no frills visa bank card, it might be 1.5%.

Across a large population of transactions it usually averages 2-3%. But if you have a B2B where all your transactions are from companies like mine, you’ll likely find it averages above 3%.

Businesses like stripe are betting that 2.9% is above the average they see across a very large population of transactions.