The incident happened at the shop in Sloan Street in Lurgan at about 2000 GMT.
A man armed with a knife and an iron bar entered the shop, handed over a bag and ordered her to fill it with money.
He then left with the money. Police are appealing for anyone with information to contact them.
Cyclists injured in hit-and-run
Two cyclists have been injured in a road accident in County Londonderry on Saturday morning.
The police said a dark-coloured car, possibly a BMW, collided with the cyclists at about 0840 GMT on the Drumgarner Road in Maghera.
Their injuries are not thought to be life-threatening.
The police have said that the driver did not stop and they have appealed for witnesses to the accident to contact them.
Agency defends MOT emission test
The Driver and Vehicle Testing Agency has defended its handling of new emission tests introduced in NI.
The agency started to use the new system this month in a bid to cut down the number of polluting vehicles.
However, motorists have complained vehicles are failing because many garages do not have the right equipment to carry out pre-MOT tests.
John Crosby of the agency said it had sent information leaflets to those coming for appointments.
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