Free entrance X chargeable entrance. Free parking X paid parking. Free service X chargeable service. Free consultation X paid consultation. Free health care X expensive health care. Free toilet X pay toilet. Free fare (in the sense of a free ticket to travel, the opposite can be a paid ticket). Here there is a mentioning of paying a ticket fare.
I checked Garner's Modern American Usage; although BG doesn't address free of vs. free from, he writes that the distinction between freedom of and freedom from is that the former indicates the "possession of a right" (freedom of speech) and the latter "protection from a wrong" (freedom from oppression). So free from is used to indicate protection from something problematic, and free of (which ...
What is the opposite of free as in "free of charge" (when we speak about prices)? We can add not for negation, but I am looking for a single word.
I don't think there's any difference in meaning, although "free of charges" is much less common than "free of charge". Regarding your second question about context: given that English normally likes to adopt the shortest phrasing possible, the longer form "free of charge" can be used as a means of drawing attention to the lack of demand for ...
For example, imagine some food company decides to make their fruits permanently free. Online, you can "order" them (for free), but in person, what do you do? What would be the professiona...