Postal workers on Trecenydd Industrial Estate called in police when they thought they had intercepted a bomb.
Trains on the nearby Rhymney line were stopped for 15 minutes on Friday morning with Aber train station sealed off.
The suspicious package was eventually deemed to be safe.
A spokeswoman from Gwent Police said: “We were called at approximately 8am this morning as the Royal Mail offices thought a package was suspicious.
“A cordon was put in place and trains were put on temporary stop as a precaution, the railway runs directly behind the building, while an investigation was carried out.
“We consulted with the Explosives Ordnance Disposal and, as a result of inquiries made by police officers, the package was examined and deemed safe.”