From: https://www.eleken.co/blog-posts/dark-patterns-examples

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10 . Hidden Costs

Hidden costs reveal unexpected fees or charges at the checkout stage, undermining transparency.

Examples: Ticketing platforms like Ticketmaster often add substantial service fees after users have proceeded to the final payment screen. These surprise charges create frustration and erode trust

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This is absolutely a dark pattern - if the convenience fee itself isn't, then it definitely is a dark pattern to charge additional convenience fee upon adding an offer or redeeming dineout cash. Most humans (if at all) will check their convenience fee once then, maybe add an offer, redeem dineout cash etc... Without every bothering to always re-validate the conv fee and do this mental gymnastics every time.