Does the word STOP appear in every text sent?

Yes, from the very first message: a short mention at the end of the text says how to unsubscribe, and any equivalent word — "arrêtez", "stop", "unsubscribe" — is recognized, not just the exact term. The request is processed within 24 h, the number enters the opt-out registry and never receives a message from you again. That registry is viewable and exportable from the dashboard, so you can prove compliance at any time. The cost of that rigor: zero dollars, and an annoyed customer who stays a customer — not a complaint.