Come to think of it, I would expect a store clerk to not even mention "bitcoin" or "bits" or any other abbreviation.
If you consider how people speak in daily life.. he'd just say "that'll be three fifty".. and you'd know he meant 3.5 mbtc by simple deduction.

E.g. you go to a McDonalds.. the sign reads 1 mbtc for a happy meal, 0.5 mbtc for a coke. "that'll be one fifty".