It is actually very easy to calculate/estimate. In FY20, US...

It is actually very easy to calculate/estimate. In FY20, US revenue was $162.7m on GMV of $3,990.4m. So an average margin of just under 4.1%. However, that revenue number includes late fees (which are obviously not paid by merchants). Total Group late fees in FY20 were $68.8m. If you assume US late fees were $25-35m (the company doesn't split late fees by region) that equates to 0.6-0.9% of GMV. This reduces the average fee paid by merchants in the US to 3.2-3.5%.

PayPal is going to charge 2.9% + 30c per transaction. If you assume the average transaction is $150, then that 30c equates to another 0.2%.

So PayPal will be charging 3.1% compared to AfterPay's 3.2-3.5%. I don't see this differential as a reason why a retailer wouldn't offer both the PayPal and AfterPay BNPL options on their website.

Also worth noting that it is highly unlikely that PayPal will reduce the fee they are charging on BNPL because it matches their regular transaction fee and would therefore have a significant cascading effect (negative) on their overall profitability if reduced.