Session messages are charged on a per message basis by the WhatsApp Business Solution Provider (BSP). The cost per session message can range between $0.003 to $0.007 depending on the BSP. Since session messages fall within the 24-hour window, Facebook does not charge the additional flat per message fee.
Template messages can be sent irrespective of the session duration. So, template messages are charged per message by the BSP as well as by Facebook. The price per template message on WhatsApp API can range between $0.003 to $0.007 depending on the BSP. An additional flat fee of $0.0085 per message is charged by Facebook. The combined cost per template message is around $0.0135.