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


It’s tantamount to being required to provide personal details in order to purchase something.
On principle, I shouldn’t be required to provide my personal details to purchase.
“But that’s your choice”
Not really… selling me an item at half price in exchange for my details is really just extortion. “We’ll charge you double if you don’t provide us your contact number”