I get shortchanged on the Île Saint-Louis

a) T-Mobile (and Starbucks) have no fucking clue what they're doing with wifi points. I'm going to be asking for refunds shortly, because their well-run operation stopped, it seems, at the crossing.
b) I'm hoping this scrolls off before my parents chance to read this, but I had my first infuriating Parisian experience today. Short version -
There's a grocer next to my parents' hotel. Since they stay there often, they know the dude, whatever.
The last few days, we've hiked over to their hotel before we all head out.
Today, we stopped at the grocer to buy water, because it's really hot here. We bought two .65E bottles of water for 1.30E. While I was working out the change in my head, my mom & wife were talking to the grocer, the grocer's joking that I can just give it all to him and he'll work it out. I'm reluctant to do this. I give him 2.30E in a two-euro coin and .30 in other change. He says "You pay big this time, next time you pay small." I stare at him, stand there, wait. He doesn't move. I stand there, hand out, expectent, and nothing. My mom & wife chat about moving on, everyone else in Paris has been nice and helpful, plus he knows my parents-- I figure it's good. Plus, it's my mom's birthday, I don't want to cause some big scene.
I stew about this for some time, in particular his refusal to hand over a euro when clearly I was not into his "wheel of karma" argument. It pisses me off.
Then later, after we've hiked around and whatever, we stop in to buy crackers, more water, etc. This is a perfect opportunity for him to give me that euro he owes me, or even a good faith chunk of it. Nope, exact change.
Bonus incident: I'm stewing some more about this, and complain to my dad while birthday mom is up in her room. He expresses consternation, not because I've been ripped off for 1E by someone he knows and does business with all the time, but in his words "I just can't figure out why he'd do that" which, to translate, is "I don't think that happened, or at least not in the way you describe it" which, to digress, was probably the most infuriating thing about my youth, which - and I freely admit this as much my fault for being delinquent all the time as anything - had any number of incidents where it was my entirely truthful word against someone (generally a teacher or other authority figure) else's story, and they'd totally buy mine. In this case, the essential implication is that I, a former cashier myself and not a s dull knife at this, must somehow be responsible.

Anyway. Fuck you, grocer on Île Saint-Louis! This is why visitors get bad impressions. Everyone else is kind and nice, and you fuck it up for them.

I'm not sure what the proper Parisian response is: graffiti or brick-through-window. I'll ask at the hotel front desk tomorrow, see what they say.