Barry Shopper and the Deathly Discounts

I bought the book, Harry Potter and the Deathly Hallows yesterday. I was in Tescos, shopping for bread and teacakes and... there they were! A pile of HP&DHs, £10 each. So I did an impulse purchase, and put one in my trolley. What the heck, I was planning to get it anyway. Thence to the checkout, where all my shopping was duly scanned and rung up. And then the fun started.
The girl at the checkout looked at me and said, "You know that the Harry Potter's only £5 if you've spent more than £50?"
"No." I replied, "That's great."
The bill was £54.
But the girl had problems.
"I can't get it to subtract the £5. I'll need to get someone..." She pressed the help button and waited. And we waited. And the lady behind us in the queue was placing her shopping on the conveyor.
The supervisor soon worked out what the problem was.
"It's got to be £50 without the book."
Ah, all was clear. When the £5 discount was taken off, the total dropped below £50 which meant that the price rose to £10 again which pushed it back to £54... and so on in a recursive manifestation of Heisenberg's Uncertainty Principle applied to quantum economics.
"So how much are we short?"
"£6."
"I could get some more teacakes." My wife had been swithering earlier about getting an extra packet, and this pushed the decision firmly into the extra packet camp. She went off to the bakery section, while I wondered desperately how to shop for a fivers worth of stuff in as many seconds. The confectionery shelves were nearest. I rushed over and grabbed a couple of packs of Kit-Kats and a box of chocolates, and returned to the checkout.
"There's teacakes coming." I told the girl as she checked my new purchases through. Then we waited. And waited. The lady behind us in the queue had by now placed all her shopping on the conveyor, and she was waiting. I bet she was thinking how Sod's Law always places you in a checkout line that gets held up by nutters.
Sod this, I thought, and headed for the bakery section, checking each aisle as I went along to make sure my wife wasn't heading down. Fortunately she was on her way back. I grabbed the teacakes from her and ran back to the checkout and slammed them down triumphantly. The girl checked them, brought up the total and tried to subtract the discount.
Then she tried again to subtract the discount.
Then she pressed her help button.
By this time, I was beginning to think I had House Elf problems. The little buggers had obviously put a hex on the till.
The lady behind us in the queue would be thinking the same thing I reckoned.
The supervisor came and tried to subtract the discount.
She stared at the screen. She must have been giving it a silent retailer's Expecto Patronum, because the second time she tried, she succeeded!
Relief all round. Soon, all was paid for, and we were out of the store.

The book's good.