Hi there,
The Santiago Post office is open from 08:30 to 20:30 (no break) on weekdays, and 09:30 to 14.00 on Saturdays. Sundays and holidays the post office is closed.
Regarding your painting paper, I would try a bookstore called "Follas Novas"
located here. They may not have it, but I think they would know were to send you. Plan B would be to ask at the tourist office (100 meters up from the pilgrims office).
Greetings from sunny Santiago,
Ivar