If you send WhatsApp messages through Meta's Cloud API, the invoice is not a flat fee and not a price per seat. Three things decide it: the category of each message, the country of the person who receives it, and, since 1 October 2026, how many service replies each of your phone numbers sent that month. This post walks through the model as Meta documents it, then turns an open CSV of the rate card into a monthly estimate in about 30 lines of Python.

The model in six rules

1. You pay per delivered message. Since 1 July 2025 Meta charges per delivered message. The old "24 hour conversation" is gone as a billing unit. A message that fails is not charged.

2. Every message has a category. marketing , utility , authentication or service . Templates get a category when Meta reviews them, Meta can change it later, and you pay the category that applies at the moment you send. Free-form messages you send inside the 24 hour customer service window are service .

3. The recipient's country sets the rate.…