Palermo end Milan double hopes
Giulio Migliaccio and Cesare Bovo, with a penalty, scored in the second half of the semi-final second leg as the Sicilians completed a 4-3 aggregate win.
Zlatan Ibrahimovic replied in stoppage-time for AC Milan who clinched the Serie A title on Saturday.
Bovo was sent off almost immediately after his goal for a high kick on Alexandre Pato as Palermo reached the final for the third time, having lost the previous two.
After sitting back for the first half, Palermo - who have never won a major trophy - came to life after the break, inspired by Argentine Javier Pastore in midfield and Uruguayan Abel Hernandez up front.
Migliaccio put Palermo ahead with a diving header from a corner in the 63rd minute before Pastore sent Josip Ilicic clear but he was hauled back by Van Bommel as he was about to shoot.
The hard-tackling Dutchman was given straight red card before Bovo converted the penalty, only to be sent off himself two minutes later.
Ibrahimovic's flicked goal came too late for Milan.