Emergency services were called to the premises in Omagh after the discovery of the package.
A PSNI spokesman said: "Police received a report of a suspicious package at premises on High Street in Omagh at 8:05am this morning (Wednesday, 8th January).
"Enquiries revealed it was nothing untoward. "
Royal Mail said staff were evacuated in line with policy and procedure and returned to work after a police investigation.
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"The office is operating as normal," a spokesperson said.