Let's use Stripe as an example, there fee is 2.9% + 30 cents.

That 30 cents - is what's often ignored but makes up a disproportionate amount of the fee.

On a $30 transaction, paying 30 cents is equivalent to a 1% fee.

So Stripe on a $30 transaction is effectively 3.9% in fees (on a $30 transaction).

Interchange (which goes to the issuer), would only be approx 1.75% (of the 3.9% fee charged by Stripe).