Store owners in the vicinity of the Ed Sullivan Theater felt the pinch when David Letterman took his Late Show to L.A. last month. Now that he's back, business is booming again. For a stunt on the June 6 broadcast, Letterman marched 50 members of the stand-by audience across Manhattan's 53rd street to the Au Cafe, where staffers had selected a menu of chicken, steak, or salmon, plus salad and dessert, for each person. ''He came over, sat them down to eat, and then it started to rain,'' says Au Cafe manager Bobbie Lloyd, who pushed umbrellas together at the sidewalk eatery to shelter the crowd. The bill totaled $1,000, with an 18 percent gratuity. Does Letterman, who often orders in from the restaurant, have an account? Says Lloyd: ''He does now.''