Dear sir/lady/kid that helps bag groceries at the supermarket:
It's okay if you look over at the magazines I'm buying, I don't mind if you flip through it real quick before bagging it, but if you start reading it over calmly and forget the 20+ groceries that need bags, don't frown/huff/pout when I don't tip you so well, the cashier and I had to pretty much do your job. I don't blame the lady that started complaining when you took too long to bag my things, you were just starting on the vegetables when I was done paying for them.
Also, don't try to fool me, I saw you placing my magazine in the little space under where you keep your bags, next time you do that I will accuse you of trying to steal from me.
EDIT: I live in Mexico, baggers are volunteers that don't get paid by the supermarket/company, some locations even have a little sign stating so and asking to tip the baggers.