This actually does happen kind of often.

I work in the restaurant industry (while going to school) as a waiter. We are encouraged to upsell as much as we can. For example, we charge extra if you want avocado in your burger or on any of the entrees. For a different type of fries we charge an extra 59 cents. So yes, this is common, but really I have never heard of charging for Lettuce before....