Where I live, we don’t yet have those cards. I despise the idea of them and hate getting caught out by them (I’m looking at you Tesco and Lidl).
Normal price for something - 0.49 (in stores without these cards) But in their store it’s only that price with their ‘club card’ or ‘plus membership’.
Otherwise it’s 1.59
There are loads of other items with high markups.
BUT - the marked price is the normal store price to the inexperienced shopper of these data collecting stores, with the blind spot being the need to have a store card to get the ‘discount’. My full shop was an extra 103.46 altogether between the two stores (51.22 in one 52.24 in the other) rushing to get christmas dinner and dining and extras. Should be banned. /rant
The government doing a sick job of protecting consumers. /s


The beer I bought yesterday was 12 quid in Tesco and at least 15 quid at all my other available options. 20 quid in one of those places, none of which has the card system. Same with a number of products I buy which is why I shop there. It’s objectively cheaper than not having a card or shopping somewhere else.
And the shockingly detailed data is in no way shockingly detailed. It’s my name, email, and some food I buy.