Which doesn't make it an externality, because they are still choosing to use the vendor. My favourite take-out place for a solid 2 years was the cheapest on the street and probably because they didn't support Afterpay or credit cards. The customer has all the power to avoid the cost if they want to.

They pay the Afterpay surcharge because they want a standard shopping experience that supports lots of different ways of paying. If they want to shop on price Afterpay will vanish from their view.