As the merchant, you will be charged a small fee per transaction: 6% + 30c (plus GST). There is, however, no instalment, set up or plug-in fee, so you will never be charged for having Afterpay until a customer uses the service.
There is no fee or interest charged to the customer whatsoever. They will receive their product immediately and have 6 weeks to pay off the transaction (four fortnightly instalments). You, as the merchant, will receive the full payment within 5 business days, and we at Afterpay will assume the full risk of non-payment or fraud.