The two youths walked into Georgetown Garage at around 10.30 pm on Friday brandishing a large blade and forced the only member of staff on duty to hand over the contents of the till.
It is not yet known how much cash they made off with.
Both robbers had hoods pulled up to obscure their faces and are thought to have been wearing masks.
The member of staff was not injured during the five-minute ordeal, but the police said that he was threatened by the pair and left ‘extremely shaken’.
