Merchants are not allowed to add a surcharge line item for using Afterpay. Instead they have to increase base pricing across the board to cover their increased costs. Afterpay are quite happy for merchants to do this, what they don't like, and what they actively police, is merchants adding a line item passing Afterpay costs onto the consumer.

The macroeconomic impact is potentially enormous. Your mission is to capture an entire generation of consumers (millennials) to use your payment mechanism, and embedded in that is a hidden cost which is comparable to consumer VAT (called GST in Australia) that the sovereign nation also levies. Eventually regulators will take notice of this massive some of money.

A comment I once read of HN as while back resonates deeply with me, that is how broken and unsolved the problem of paying for things electronically remains, in terms of costs, reliability, security and ease of access. Inspite of all the incredible transformation that has occurred over the past 30 years. You think about all the money that moves around for electronic payments, in an ideal world, the net levy should be a fraction of a %, not the 1.5%-3.0% that is charged by credit cards or the 6-8% that BNPL charge.

BNPL is a seemingly a separate situation since it requires the company to front money to merchants with the risk of the person not making a payment and defaulting on the loan, but the merchant fee is indeed where it seems deceptive, even if it might mean more sales due to said customer deception of "0% APR".

What if you do something like a “cash discount?”

It means that can't price discriminate specifically against Afterpay as a payment form. The merchant is free to set prices to whatever they want (as OP notes: the cost of the expensive Afterpay service will be borne by all customers), but Afterpay can't cost the customer more.