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 have until 11pm the following day to make payment. After 11pm the following day, if that payment has not been made, 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).