According to, (scroll to the end ) https://www.septakey.org/info/fare-products there should be a "Quick Trip — Regional Rail — All Other Lines, Paper ticket purchased from a Center City Station Fare Kiosk " that you can buy yourself. I dont know when it expires.

But thats only for zone 1-> outlying stations.

Or just buy at the station you are departing from? Buying elsewhere is just complicating things.

Inbound pay the conductor cash.

But really the answer is a reloadable septakey card.