For AfterPay, a late fee is charged when an installment for an order is not paid after the applicable grace period (usually 10 days unless otherwise noted on your payment schedule).
AfterPay will only ever charge one late fee per installment and the total amount of late fees charged on an order will never exceed 25% of your initial order value.
As soon as you miss a payment, both AfterPay will immediately stop you from making any further purchases using their services. This is because of their commitment to the high standards they set for responsible spending.
To keep you in the loop, AfterPay will send you an email notification when a payment draws near.