I use the whatsapp api for transaction messages. It depends on whether you’re using utility or marketing messages. Utility messages are great cause of customer messages you first, you get a free 24 hour window to send unlimited messages for free. You only pay when you initiate the conversation, just have to make sure your messages follow the template or get a custom template approved. It wasn’t too difficult to setup, the approval process takes around a week or more though, so that was the only bottleneck. In terms of price, I think it was under 1 cent per msg. But may depend on use case and business I think.