The John Kamau-led team went down to goals scored by Michael Madoya and Boniface Muchiri who scored his 12th goal of the season from the spot.
Posta Rangers goalkeeper Gradus Ochieng brought down David Majak and referee Peter Mwai awarded the Brewers the penalty.
The Mailmen's captain Jockins Atudo salvaged the only goal for them at Ruaraka Stadium. The last win for Posta Rangers came on May 9 against Ulinzi Stars and they then went on to pick just three points from a possible 12.
In the concluding season, they managed to score 28 goals and conceded 36. After the appointment of John Kamau as head coach to replace Sammy Omollo, Rangers improved where they also hammered Mount Kenya United 6-2 on April 14.
They will face the side that will finish third on the National Super League whose season is yet to be concluded. Nairobi Stima are the current holders of the play-offs spot in the NSL where they have 71 points after 35 matches.