I don't know what that is, I'm afraid.
Many sites want payment details in the hopes that you'll be persuaded, at some point in the future, to purchase their wares instead of just taking advantage of the free ones...
they want something to charge £0 0p to. If giving card details is the "price" of a free book, that seems fair enough; you don't have to take up the offer if you don't want to.
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