Any accessory use on the same or on a contiguous lot customary and incidental to any use permitted in this district, including but not limited to parking decks and garages on a lot containing office uses of fifty thousand (50,000) square feet or greater, convenient shops selling food, medical supplies, stationery, dry goods and other similar products shall be permitted, provided that such uses shall not occupy more than five (5%) percent of the gross floor area of buildings located on the lot, and further provided that no advertisement of such uses shall be placed so as to attract the general public.