Follow carefully:
1. Can a person make kiddush on Friday after plag? Yes.
2. If one makes kiddush after plag is it shabbos for him? Yes.
3. Is there an issur melacha after making kiddush mi’beod yom? Yes.
4. Is saying “good shabbos” (once it is time to make kiddush) a kiddush? Yes.
5. Hence, saying “good shabbos” = kiddush, after plag, makes an issur melacha.