Police are visiting post offices across the county to warn them about possible attacks from a gang of raiders.
Detectives believe a series of robberies could be the work of the same group of men.
They have also linked the crimes to similar ones in Kent and South London.
Martins newsagents in Crawley Down was the latest to be targeted last Thursday.
Three men wearing scarves over their faces threatened staff and stole cash at about 5.30pm.
A female member of staff was slightly hurt but did not need hospital treatment.
The raid follows similar gang attacks on post office branches in Cuckfield, Balcombe, Horsted Keynes and Turners Hill.
Anyone who notices anyone acting suspiciously near post offices should call the police.
One man has been arrested in connection with the raids and released on police bail.
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