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Not Flixbus but Flixtrain; when I buy a Flixtrain ticket from Berlin Hbf to Köln, there's no add on is there? I'll probably have a 24h ticket from the previous afternoon anyway.
I'm assuming you're wondering about a Flix equivalent to the DB CityTicket add-on? Flixtrain (like Flixbus) is strictly point-to-point, no zonal addons included.