Grocery shopping has much weird psychology to it. The one-way doors, the music, the dairy in the back. The allure of the displays at the end of the aisles. It doesn’t all make sense, but that’s people for you.

Add the nostalgia of familiar brands; reach for a box of Maypo and I’m back in the Pick & Pay with my mom in Berea, Ohio, in 1966. The satisfaction of food. The dizzying abundance. It’s never as simple as picking up a loaf of bread.

I haven’t even mentioned price. As a successful man of the world, I seldom pay attention to prices. It’s a supermarket; whatever I buy here is going to be far less than the steak sandwich at Gene & Georgetti. The fact that I’m food shopping at all is sacrifice enough; don’t ask me to cut coupons too.

So Toni Preckwinkle’s sweetened beverage tax almost blew past me. My heart wasn’t awash with sympathy for anyone upset over an additional 12 cents for a can of soda. If that 12 cents helps weave together the fraying social safety net, well, happy to do my civic duty.

OPINION

Then my wife came home Sunday waving her Sunset Foods receipt. Dasani sparkling flavored waters, on sale, three eight packs for $6.99. Plus the new Cook County sweetened beverage tax of $2.88.

Quite a lot really.

And the kicker is, water that is flavored but unsweetened shouldn’t be taxed at all. An obvious mistake.

But stores make mistakes. As a loyal customer, I try not to judge harshly. On Monday I hopped onto my Schwinn cruiser and pedaled over to Sunset, already imagining their sincere embarrassment. “We’re so sorry Mr. Steinberg,” Mr. Whipple would stammer, fingers twisting at his sternum. He’d ding open the register, peel off two singles and a handful of change. Neighborly smiles all around.

Nope. Sheila greeted me as if I had been caught tucking steaks under my shirt. An icy call to the manager. Dasani is on the list. I point out that the drink is unsweetened. She said it’s something to do with the flavoring.

“Keep your receipt, and if you find evidence the tax was charged incorrectly bring it back,” she said.

Maybe I will do that. Two paths here: Cook County and Coke, which makes Dasani. Emails to both, holding a little race to see who gets back first. I place my bets on the corporate giant. But the relatively puny governmental entity beats them handily.

“If a ready-to-drink beverage is sweetened, it’s taxed; if it’s not sweetened, it should not be taxed,” wrote Frank Shuftan, communications director for Toni Preckwinkle’s office. “It’s very simple, and if any merchant has a question they should contact the county’s Dept. of Revenue. Unsweetened LaCroix (or similar product) should NOT be taxed. Water should NOT be taxed.”

In an ideal world, yeah. But we are not living in that world.

Armed with Shuftan’s email, I hurry back to Sunset for a long, frustrating conversation with assistant manager Charlie. He isn’t interested in looking at my email. The take-away, as best I can understand it, is: “Tough. We’d rather charge you the tax incorrectly now and refund it later than have to dig into our own pockets for this.” Though he was very nice about it.

Curious, I head to the Mariano’s a couple of minutes down the road. They offer me free tastes of smoothie, fresh orange juice and cake (more psychology) and, most significantly, sell an eight pack of Dasani sparkling lime for $3.99, plus the correct 9 cents tax.

On a roll, I swing by Jewel on Willow Road. There I buy two eight packs for $7, plus the correct 16 cents tax.

Mistakes happen. Walgreen’s did the same thing, over the same product, and is now being sued in a class action lawsuit.

And here’s the interesting part, circling back to psychology. Even improperly adding the tax, I paid $3.29 per eight pack of Dasani sparkling water at Sunset, verses $3.58 at Jewel and $4.08 at Mariano’s. So the eight-pack at Sunset, where I felt ripped off, cost less than at the other stores.

As I said, a lot of psychology involved with grocery shopping.