Best thing to do is buy this at the station here. Will need a ticket from either Wengen or Grindelwald to Jungfraujoch, as the STP is not valid beyond those places. If you want an interesting trip do a round trip, go up via Grindelwald, and return via Wengen and Lauterbrunnen.
And just get your ticket the evening before, or the morning of your trip at the station. Do not buy mountain excursions until you have seen the weather.