Three masked men used baseball bats to threaten post office staff before escaping with cash from the safe.
The men, all in their 20s and wearing dark clothing, burst into the post office in Bookhill Road, Copthorne, at 12.50pm on Friday.
Staff were forced to open the safe and an undisclosed sum of money was taken.
The trio are thought to have escaped in a black Nissan Sunny, which was later found abandoned outside the Shipley Bridge Hotel.
Police are appealing for anyone with information about the raid to contact them on 0845 6070999.
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