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.