As the merchant, you will be charged a small fee per transaction: 5.5% + 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 when your eftpos terminal settles for the day, and we at Afterpay will assume the full risk of non-payment or fraud.