At about 2.30pm a man went into the budget store in Halls Road and selected a bar of chocolate.
He then went to the till to pay and when it was opened he demanded money from the male cashier.
He took a quantity of cash and left the store on a bicycle.
The robber is described as in his late 40s, white and about 5ft 10in. He was wearing a black hooded top, a black coat, blue jeans and dark shoes. He also had a black woollen hat and gloves.
Police want to speak in particular to a potential witness, a person who was sat in a car parked in a disability bay outside the shop. The red Vauxhall Corsa drove off between 2.30-2.45pm. They believe the driver may have information to help them and urge them to get in touch.
Anyone able to help is asked to contact South Gloucestershire CID on the force number 101.