1. Rede Expressos buses from Lisbon only depart from the Lisboa Sete Rios Bus Station. They do not leave from or pass by the airport.
2. To get from Lisbon Airport to Lisbon Sete Rios Long distance Coach Station take the following bus which leaves from outside the Arrivals hall:
(Aeroshuttle - Copy of note from Lisbon Airport website.) Aeroshuttle (CARRIS Nº96) - Available everyday, every 30 minutes between 7am and 11pm, the aeroshuttle connects Gare Oriente - Airport - Entrecampos - Sete Rios Bus/Coach Station - Praça de Espanha.
3. For online purchase have a look at this discussion by Cubsur on tripadvisor forums.
http://bit.ly/bXqWj7
As far as I am aware there is no difference in price whether tickets are purchase online or in persons.
