Billing & Credits
Meta does not charge for both sending and receiving messages individually. Instead, it charges based on a 24-hour conversation window. Once a conversation starts, you can exchange unlimited messages within that 24-hour window without incurring additional charges.
If the 24-hour window closes, a new conversation will start, and you will be charged based on the following scenarios:
Business-Initiated Conversation: If you send a message to a customer after the 24-hour window has closed, Meta will charge you for a new conversation under the applicable category.
User-Initiated Conversation: If the customer messages you first and you respond within 24 hours, it will be considered a user-initiated conversation. However, if you respond after the 24-hour window, a new conversation will start, and relevant charges will apply.
Meta charges for WhatsApp conversations based on the following factors:
For example, if you want to send marketing messages to your Indian clients, you would select Market as India, Currency as Indian Rupee (INR), and Conversation Category as Marketing. In this case, the applicable Conversation Rate would be ₹0.8631 per conversation.

To get detailed pricing for your specific use case, visit the official Meta pricing page: Meta WhatsApp Business Platform Pricing
This will display the exact rates applicable to your conversations.