An armed robber made off with £40 from an Essex post office after wielding a plastic bag concealing a gun-shaped object.
The man demanded cash from two female staff at Brentwood Road, Herongate, Brentwood, on Monday afternoon.
Police believe he may have had a vehicle nearby, possibly a Kia Sedona.
He is described as white, of large stocky build, about 5ft 10in (1.78m), with shaven, light hair and is in his late 30s to early 40s.
He was wearing a dark hooded tracksuit and gloves.
Anyone who saw the man before the incident or after the robbery at 1610 BST is asked to contact police.