Afterpay does not charge a fee to shoppers when purchasing. The only fees applied to a shoppers account are late fees if your scheduled payments are unsuccessfully processed and, after being notified, you do not log in to your Afterpay account to make your payment via a different method.
If you have not made the payment before the due date, and the automatic payment fails, you will incur a late payment fee of $10 and a further late fee of $7 if the payment is not made within 7 days (as stated in the Afterpay Terms).