It isn't true that retailers take a tax write off when you donate at the register. The money you donate is a direct donation with no strings attached. You yourself are allowed to itemize it as a donation when you do your taxes.

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It is 100% a scam to tell someone you are raising money for a charitable cause when you aren't. You got a candy bar, but you believed your money was going to a charitable cause when actually it was not. You likely would not have bought the candy bar had you not been lied to about the money going to a charitable cause. If that isn't a scam, I don't know what is.