A postmaster was threatened with a knife and had a paper bag taped over his head by two men raiding the Hacheston post office.
The robbers then shut him in the toilet.
The incident happened shortly before 0900 BST on Friday.
One of the robbers is described as white, 5 ft 11 ins tall and was wearing a blue fleece, blue tracksuit bottoms and trainers.
The second man was the same height and wearing dark clothing.
A spokesman for Post Office Ltd said a trained member of staff has been supporting the victim, and there would be ongoing support if necessary.
Suffolk Police are appealing for witnesses.