Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Bangkok to San Sebastian as you normally would.
For a flight from Bangkok to San Sebastian, you will be flying into San Sebastián. San Sebastián (EAS) is just 15.7 km from the centre vof the city. San Sebastián is the only San Sebastian airport and it handles 0 inbound flights from Bangkok per day. 0 airlines have flights to San Sebastián from Bangkok on a regular basis.