We have traveled with 3 of my daughter's friends (at different times- I'm not crazy LOL). In all cases I got the notarized letter with permission to travel, make medical decisions and to sign all waivers. In the case where the parents were divorced I got one from each parent. We paid cruise fare, parents paid air/walking around cash. Only 1x in Jamaica the friend was pulled aside by port authorities and asked who we were to her and if she was safe with us. Still glad I had all the docs. They were 12 on that trip. We had known the girl her whole life so no need to go into expectations. They knew they were lucky and weren't about to blow it!