Before the match, Cartagines were one point clear of fifth-placed UCR with two games to play and they got off to a perfect start after five minutes when Erick Scott found the back of the net.
But midway through the half, Zeledon fought their way back in to the match as Andres Castro equalised against the run of play.
Cartagines' dominance paid off six minutes after half-time with Scott securing his brace and the match-winn.
UCR's 2-0 loss to the already-finals-bound Alajuelense means Cartagines cannot be caught in fourth position and will feature in the final stages.
Herediano wrapped up top position with two victories in four days; a 4-1 thrashing of Limon and 3-2 victory on the road against Perez Zeledon.
Victor Nunez grabbed a double in both matches as Herediano, who took on Zeledon in a previously postponed encounter, firmed as favourites to retain their championship crown.
In other matches, Deportivo Saprissa suffered a hitch before the finals after a 1-1 draw against rock-bottom Belen Siglo XXI, Santos de Guapiles thumped Carmelita 4-0 and Uruguay came from behind to defeat Puntarenas 2-1.
