A woman was threatened with a gun during the incident, which took place on Friday night, before the assailants escaped with cash and cigarettes.
Chris Lyttle MLA said: “This incident must have been a horrific ordeal for staff present at the time and I must condemn it. My thoughts are with them.
“I would appeal for anyone with information to come forward and contact the PSNI as soon as possible. Those responsible must be removed from the streets before they can strike again.”