The incumbent is Mindshare, which did not repitch for the business.
The Post Office called the review at the beginning of the year and shortlisted agencies in April. The process went through the Crown Commercial Service.
The review was part of the public procurement policies that the Post Office follows.
Mindshare won the planning and buying business in 2012 when it was reported to be worth £12 million. Before this it was handled by OMD.
Carat handles the media buying for the CCS.
A Post Office spokeswoman said: "We are currently completing our internal approvals and once that process concludes will be able to confirm the outcome of our recent pitch process."