re: Travelling to Paris for 15th Wed Anni

Yes there will be a place to purchase tickets, there's one at basically every train stop and certainly at the airport. The airport will usually be extra though FYI, I don't know CDG prices but for 2 people it can sometimes be close to the same for a taxi/ Uber depending on where you are going.

As said, you can buy unlimited passes but often times you won't need that. You will just take it 2-4 times a day most days and that can often be cheaper to just pay by the trip. Given that, it's sometimes worth the extra euro or two a day to just buy the unlimited multi day pass and not have to worry or take the time to replenish.