The crime happened about 10:30 p.m. at 261 N. DuPont Highway (U.S. 13).
The man pulled a black firearm from his shorts and pointed it at employees, Master Cpl. Mark Hoffman said, after handing them a note demanding money.
The workers turned over an undisclosed amount of cash and the man fled south from the store.
He was described as white, heavy set with a gray T-shirt, gray shorts, dark sneakers and glasses.
Police asked anyone with information to call 302-736-7111.