Police have released a CCTV photo of a man they want to speak to in connection with the incident on July 30 in Hazelmere.
Two men entered the store at around 11am and loitered around the self-service tills before grabbing a cash bag from an employee and running out of the store, followed by the second man.
The suspects are described as white, the one pictured is in his mid 20s, of athletic build, with short fair cropped hair and wearing a white t-shirt, grey shorts and a bandage on his knee.
The other was in his late teens, skinny, wearing a dark jacket and baseball cap.
The employee was aged in her 50s, received an injury to her arm and was left shaken up.
The men got into a blue Vectra, driven by a third man, and fled the scene – but the money bag was empty.
A 30-year-old man from east London has been arrested on suspicion of robbery and bailed until September 25.
Anyone with information is asked to contact Dc Greg Fairbairn at Bailson CID on 101, or call Crimestoppers anonymously on 0800 555111.