If you're getting the Plus package the first two people in a cabin have to have the same thing. So make sure the 18 year old is passenger number 2. For the 17 year old , as passenger number 3, you could wait to decide once on board. We have a somewhat similar situation except that passenger number three is 9. MY wonderful TA also observed that if we wait until onboard we can use OBC to pay for her package if we decide to get it.