I don't have to pay for any message if it is within the 24 hour window?
Yes, if the customer initiates messaging with your WhatsApp business account, WhatsApp will not charge for any messages (including Templated Messages) that your WhatsApp business account send to that customer for up to 24 hours following the last message that the customer sends to the WhatsApp business account ("Customer Care Window"). Any additional message being sent to that customer beyond the Customer Care Window must be a Templated Message, for which will be charged by WhatsApp.