Which of the following is more natural for a customer to use at a coffee shop?
a. Do you offer free refills on coffee?
b. Is your coffee bottomless?
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bottomless in this context is late 20th century ad-speak. It isn't really a "natural" idiom that has been used for centuries.
P.S. Although I agree with Weather Vane that it is the pot (or the mug) that is bottomless, you might well see a poster or a billboard advertising BOTTOMLESS COFFEE at a donut shop or fast-food restaurant.
P.P.S. A speaker might ask the question in either way. It is not uncommon for speakers to use the ad-speak that saturates commercial culture. "Free refills" would be understood anywhere. If you were to ask "Is your coffee bottomless?" in an area which is not saturated with ad-speak (they are few and far between) you might get a strange look.