Around 10 panes of glass were been smashed and the door was also damaged.
Branch manager Jill Hughes said she was shocked to discover the damage when she went into the office on Monday.
"It was a real shock, there was glass literally everywhere," she said.
"There was a lot of damage.
"The head office is dealing with the costs involved."
Police are investigating the incident of criminal damage.
They are unable to say whether the vandalism is connected to England's defeat against Germany in the World Cup on Sunday afternoon.