Planning on making a day trip to Segovia. Re purchase of AVANT tix - must you get reservations in advance, or can you purchase at the station without a problem?
I don't see why buying at the station would be a problem.
Purchase at the station...be sure to leave enough time in case there might be a line at the booth...
There's also a good, cheap bus option on Sepulvedana bus lines from Principe Pio metro Station (lower level buy tickets at booth window): 20-25 minute walk from Puerta del Sol – about an hour to Segovia non-stop. The first stop in Segovia drops a few passengers – the next (last) stop is at the station right in the center, about a 5-10 minute walk to aqueduct. Avoid morning and evening rush hour in Madrid, and you'll sail right out and back.