We went to breakfast at a restaurant where the "seniors' special" was two eggs, bacon, hash browns and toast for £1.99. "Sounds good," my wife said. "But I don't want the eggs."
"Then I'll have to charge you £2.99 because you're ordering a la carte," the waitress warned her.
"You mean I'd have to pay for not taking the eggs?" My wife asked incredulously. "I'll take the special then."
Although this is in the Jokes section, I've actually encountered similar cases in real life - "Meal Deals" in Motorway/A-road petrol stations where it's been cheaper if I add a packet of crisps to my sandwich/drink purchase.