WhatsApp regulates when and how you can send messages to your customers. This limitation is enforced by Meta (parent company of WhatsApp) to improve the customer experience for WhatsApp users.
All conversations are measured in fixed 24-hour sessions. A conversation starts when the first business message in a conversation is delivered, either initiated by the business or in reply to a user message. Businesses and users can exchange any number of messages, including template messages, within a 24 hour conversation session without incurring additional charges. Each 24 hour conversation session results in a single charge.
Outside of a 24-hour conversation session, you may only send an approved template message to the customer. Any message that doesn't match a pre-approved template is considered a free-form message.
To learn more about WhatsApp conversations and pricing, click here.