Natalie Haynes: Stores need to help us with maths

I've never liked multi-buy offers in the supermarket. I'm not against 3-for-2 in principle, you understand. I like it in Boots, on vitamins and shampoo, and I used it yesterday when I bought three rolls of wrapping paper in a futile bid to convince myself that I am remotely planning for Christmas.

But in supermarkets, I am suspicious of them: rightly so, it turns out. Which? has discovered that the prices of roughly one in 10 products go up when they're in a multi-buy deal. One example was of a pack of four yoghurts, usually retailing for £1. It was put onto a multi-buy deal (two for £2), whereupon the cost of one shot up to £1.59. The multi-buy saved you nothing. And if you don't have time to eat eight yoghurts before the use-by date, too bad.

Even when multi-buy deals do save me money, I'd prefer it if they just discounted the items individually. I don't have a car, so carrying quantities of anything heavy – Diet Coke, wine or gin – is a nuisance. I'll be back tomorrow for more (not for more gin. Well, not usually), but I'm trying to avoid curvature of the spine.

Then there's the financially illiterate deals that Which? discovered, including packs of sweets for 34p each or four for £3. Of course, the supermarkets all say the information is on the shelf for customers to check that they're getting the best deal, but who goes shopping to practise mental arithmetic? Apart from me, obviously: I love sums. But I'm not trying to buy a week's shopping and ferry children at the same time, so I can afford to indulge in extra maths.

If the supermarkets really meant what they said about trying to help customers get the best deal, they would offer a walk-round service. You could book an appointment with a chap carrying a calculator and, ideally, wearing jaunty spectacles. He'd have to do the maths for you, and show his workings.

If he can't tell you the cheapest product in under a minute, you get the baked beans for free, and a personal letter from the managing director, apologising and offering to deliver your groceries for a month to make it up to you.