Setup vs running cost
You pay once to set up and build the flows. After that, Meta bills each delivered template message: a booking confirmation or reminder is a utility template, a promotion is a marketing template. Anything you send in the 24 hours after a customer messages you — including utility templates — is free.
Why use the official API
Grey-market bulk senders are cheaper but get numbers banned. The official API protects the number that is your customer relationship, which is worth paying Meta's per-message fees for.