After hours pickup is incredibly rare in the US - the point that I'd almost say it's non-existent. As a part of the pick-up process the rental company will need to see your Drivers License to confirm you ID, and this is something they can't do with a drop box.
I have heard of some rental locations that are located within hotels allowing you to pickup the keys from the hotel front desk after the rental cars office is closed, but even this is rare.
However rental companies are most major airports are open 24/7 or at a minimum very extended hours (based on the airport hours), so as long as you have a way to get to the nearest major airport you should have no issues. For Pittsburgh specifically it appears that the majority of the rental companies are open from around 6am to 1am.
After-hours drop-off is more common, and whilst not all locations allow it many do - although in many cases the rental will only be considered completed once the office opens (which means you can still end up being charged even after you've returned the car).