You're forgetting the millions of lead referrals that Afterpay...

You're forgetting the millions of lead referrals that Afterpay provides. Why is this important? Because merchants need to spend money on marketing for each extra dollar they make. So PayPal charging 2.9% for their services means that for each dollar that the merchant makes through PayPal, you have to add advertising costs to the 2.9% surcharge.

Afterpay charges ~4% (more or less depends on the merchant) and marketing is essentially provided. So it's important to look at the differences in fees charged in the wider context of the merchant's business.