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Stevia is generally sold and labeled as a supplement, not a food, so that's why Sams Club can list the ingredients as "also contains maltodextrin". It follows the guidelines set by the FDA for supplements. You are buying stevia, so they have to tell you how much stevia is in each serving but they don't have to tell how much of other ingredients are in there - they just have to name them.
It's pretty common for manufacturers to use maltodextrin as a 'flowing agent' for stevia. Not all brands do, though, so read the labels if you don't want to consume maltodextrin.
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