However, there are two services a day, every day and I am sure you will have no problem arriving at Estacion Plaza das Armas in Seville and buying a ticket there are then.
I used the 08:20 bus last Friday from Faro to Seville, and it was only about 20% full.
Have a look at Cubsur's page here for more info:
http://thomasallen.home.sapo.pt/travel/001.htm
You are going to have great difficult getting back to Madrid Airport on time if your flight is early morning on December 22nd. You do not say what time your flight is. Be aware that Madrid Barajas Airport is a way out of town. You may need to sit the night at the Airport.
Your best bet would be to take an EVA Transportes Bus from Lagos to Lisbon Sete Rios, early on 21st December, 2008 and then connect with one of the buses operated by alsa.es for Madrid. Be aware that Estacion Sur Bus Station is on the South side of Madrid and you will then need to use the Metro to get to the Airport.
Timetable:
Lagos to Lisbon (Eva Transportes)
Lagos Lisbon
05:45 09:30 Weekdays
07:45 11:30 Daily
10:15 14:00 Daily
12:45 16:30 Weekdays and Sundays
15:30 19:15 Daily
18:15 22:00 Daily
Lisbon Sete Rios to Madrid (Click to enlarge)- www.alsa.es
