The incident occurred about 2:40 p.m. Thursday, Richmond County dispatch said.
"He acted like a customer looking for assistance," sheriff's Lt. Calvin Chew said.
The clerk gave him an application for a phone. Then the man demanded cash from the register while brandishing a silver handgun. He ordered the clerk to lie on the floor in a rear room.
The suspect was described as a black man , 6 feet tall with a medium build, wearing a gra y oversized T-shirt, dark pants and an orange baseball cap.