I'm going to complain about service in the grocery store
Well now you have me intrigued, what about this grocery store are you planning to complain about?
Nothing has gone wrong and I'm complaining because I want to vent before I get angry.
It's not nice to try and get someone in trouble who is just trying to do their work. You need to calm down.
It's nice to complain about these things so I can give these places better service next time.
You just said that you have nothing to complain about, so you're not helping the service but rather hurting the people who work there and I think that you should seriously reconsider.