So I parked today outside Commons building at 6 PM (and paid $5), it says on the ticket that it expires on 9PM. but it also didn't say anywhere that we can't park overnight?

well there are people that do but i wouldnt recommend it. sometimes they check... and they'll give you a ticket. You should get permission from the student center.. secuirty office. One of my freinds got a pass to park free overnight when ever.