“Free” inside an economic context, is short for “free.” As such, it’s right. To express one thing is not incorporated (in the event that, such as, popcorn just weren’t no-cost, even after ticket) you can state ‘The popcorn is not included in the citation price’. If you are writing about something, it should be more common simply to explore a term such “hence have to be purchased”. If not, it is common to use a term such “entry charges can be applied”, “subject to fee” an such like. You need to check if it suits your own context, even in the event.