Derbyshire Police officers were called to Crich Post Office on the Market Place at around 12.30pm on Wednesday, April 26, to reports of a robbery.
It was reported that two men entered the shop and threats were made towards a member of staff.
Cash was stolen and the men left the shop and made off on a motorbike.
A 35 year-old man has been arrested on suspicion of robbery and remains in police custody. A 28 year-old woman was arrested and has been released under investigation.
Anyone with information is asked to contact DC Thor Long on 101 quoting reference number 17000173454.
Alternatively, send him a message online by visiting the Contact Us section of www.derbyshire.police.uk/Contact-Us.
You can also call Crimestoppers on 0800 555 111.