I believe they eat the loss when the customer defaults, though.
It’s basically the app version of offering interest free payments on a purchase.
[1] Some gas stations have a different cash rate to pass the savings on to the customer
It's a textbook example of an externality.
No it isn't. It's literally against their TOS for merchants to add on a surcharge for using Afterpay.
If you don't want to pay the surcharge, shop somewhere that doesn't offer Afterpay. Or somewhere that doesn't honour the TOS, I once got an extra egg in my soup for paying cash.
Yes, by all the customers, including those that don't use Afterpay.
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.