The robbery happened at about 11.50am on Friday, March 29 at the post office on Wakefield Road, Fitzwilliam.
A spokesman for the force said: "Two males with weapons entered the premises and made demands for cash.
"The assistant handed over a quantity of cash and the suspects left the scene in a blue Ford Focus."
A 41-year-old male has been arrested in connection with the incident and police have recovered what they believe to be the suspect vehicle.
Police say one of the suspects was said to be wearing a balaclava, a long sleeved top and Nike trainers.
The second suspect was larger in build wearing an anorak with the hood up, a balaclava, and gloves.
Neighbourhood officers will be in the area today (Saturday) today to reassure the community.
Anyone who witnessed the incident or who may have further information is asked to contact Wakefield CID on 101, quoting crime reference 13190162698 or use the live chat facility
Those wishing to remain anonymous can also pass information to the independent charity Crimestoppers on 0800 555111.