It's time to play... PASS THE BLAME!

I was making a delivery to "XYZ Pharmacy" today. This chain of drug stores receives and controls its inventory by scanning product UPCs into a scan-gun computer, which interfaces with the store computer, which hooks up with the corporate machine. When an item is scanned in as being received into store inventory, the cost is calculated to prevent overcharging or conflicts with vendor invoices.
All that said to prove a point: Big Brother does NOT come without flaws.
My delivery today was simply for 3 cases of 2-liter bottles of soda. The cost per case is $9.60, making the total on my load $28.80. Upon scanning in my stuff, the manager said, "Whoa. There's a discrepancy of over $5."
"On 3 cases?" I doubted. "Something must not have entered in correctly. Why don't you verify the products and costs?"
...Which she did. I watched the data on the scanner screen to see if there was a system error. ...And there was...
Yep, 7Up, 1 case, cost $9.60. Canada Dry, 1 case, $9.60. Root Beer, 1 case, $9.60. Scan total: $23.04???
OK, so the computer doesn't add correctly. But what was completely maddening was the attitude I got from the manager.
"There's something wrong on your invoice," she jabbed.
"No, there's not," I retorted. I watched the screen as you verified the items. $9.60 3 times equals $28.80, and the gun read $23.04."
She wouldn't give up her new math. "No, there HAS to be something wrong. We're getting overcharged by you."
I'm all for being patient, but not when dealing with blunt stupidity, haggling over 3 cases when I still have 17 more stops to make. "Look," I forcefully exclaimed, "Do you want to figure this out on a piece of paper? $9.60 times 3 is $28.80, NOT $23.04. Your system is screwed up. The error is in your system, not on our charges."
Not deterred, and clearly unwilling to be obviously corrected by some poor slob of a delivery dude, the store manager quipped, "Never mind. I'm just going to have to contact the district manager to tell them how you're overcharging us. Just put the product in the back."

*Voice of Lundberg from "Office Space"* ...Yeeeaaaaahhhhhhhhhhh...

I'm guessing that A) this gal's strong suit is not mathematics; B) this gal's strong suit is not business administration; C) this gal's strong suit is not people skills; D) this gal's strong suit is not taking responsibility for her mistakes; E) All of the above.