Between 6.40pm and 7pm on Saturday 7 November, a member of staff at the Ashwii Mini Market, in Carnation Road, is said to have been threatened with a knife by an unknown man.
The offender demanded they hand over money and left the scene on foot after being provided with cash from the till.
Forensic enquiries are ongoing and a review of available CCTV has taken place.
Officers have also obtained information from several witnesses and are now in a position to release a description of the suspect.
He is described as being white and around 5 feet 6 inches tall. At the time he was wearing a black tracksuit, a black hat and black Nike trainers.
He was also wearing a white and black scarf across his face and was carrying a brown bag.
Anyone who saw anything suspicious at the time of the offence, or who believes they can recognise the offender from the description provided, is asked to call Kent Police on 01634 792209 quoting 46/199987/20.