There aren't any cup a soups that are free, you're generally looking at 4 Syns minimum.
The closest thing to a cup a soup is the Mug Shots, some of which are free.
You have to make sure they're the official ones from the proper supermarkets though... The ones from the cheap shops (B&M, etc.) aren't made the same way. They're made specifically for the cheap shops so there are Syns in them.
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