The fee on Afterpay orders is 4% ex GST (4.4% inc) - this means you won't pay a card fee on these transactions as it's processed through Afterpay. You will only pay the Afterpay fee and your Mr Yum fee as normal.
For example, if your venue has 2% Mr Yum fees and usually passes on the 1.7% Stripe fee to customers, the fee on an Afterpay transaction is 4.7% (2.0% - 1.7% + 4.4%). Fees can't be passed onto the customer as 'surcharges' in the way a card fee usually can be.
You get your payouts as normal because Afterpay ‘pays for the order’ on behalf of the customer until they pay 4 fortnightly instalments.