Google "sixth Penn garage prepaid parking", find their website, read that you are supposed to call the cultural district, then park somewhere that doesn't charge $15 on Sundays.
I am not going to be any help, but I do have a suggestion. Maybe you can pay as you park, or pay in the lobby as you are walking out. That way you can avoid having to wait in lines when you are leaving.
Oliver garage is really cheap on weekends, and a close walk.
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