For your information the free 3 hours parking for Aeon Big Subang Jaya has changed, now they offer free 1 hour parking for non member of Aeon Big, Subang Jaya. Free 2 hours parking for member of Aeon Big, Subang Jaya. So you need to use the member card to swipe at the machine, haven't try yet the card.
About the Post office in Aeon Big Subang Jaya, here's info on their office hour during this Ramadhan.
Mon to Sat
9.30am to 5pm
11am to 3pm
Closed on Public Holidays.
We went to SS15 Post Office and SS13 Post office they are not open in the afternoon. So I know it's best to go the Aeon Big, Subang Jaya.