The Shulchan Aruch says that a store is not Chayav in a Mezuzah. The Taz explains that since the proprietor is only there during the day and doesn't spend his nights there it is Patur from a Mezuzah. However if there is merchandise in the store that is kept there overnight it is Chayav in Meuzuah. Even a storage room is Chayv in Mezuzah, certainly a store which is used not only for customers but also for the storage of merchandise is Chayav in Mezuzah. (Sefer Yad Ktanah Al ha'Rambam)
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