The robbery took place shortly before 12.45pm on Friday, 19 July, at the Western Union branch in Finchley Road, NW3.
Two men entered the store - one brandishing a handgun, the other armed with a sledgehammer.
The suspect holding the gun threatened to shoot staff and demanded they hand over money.
Meanwhile, the second suspect repeatedly struck the glass screen that separates staff from customers with the sledgehammer.
Between £8,000 and £10,000 was given to the men by a terrified staff member.
They then ran south down Finchley Road before turning left into Fitzjohns Avenue. They then got into a waiting car, a silver Renault Laguna, and drove away north, in the direction of Hampstead.
More top news
Police are looking for a man who sexually assaulted a 12-year-old girl at a London train station yesterday while she was with her family.
A gang of robbers who dressed in burkas to stage raids on high-end stores in London have been convicted today.
The retrospective is so in-demand the museum will open its doors all through the night for the first time since it opened in 1857.