The main site to get Post Bus information is http://www.fahrplanfelder.ch/ with a search box to access any relevant timetable. The Post Bus driver normally has timetables available on the bus. Here are various 2010 timetables:
If you go over the Golitschenpass to the Elsigenalp and take the Panoramaweg to Frutigen, you can catch the BLS bus from Frutigen to Kandersteg at the Rybrügg bus stop by the Tellenburg viaduct, rather than walking all the way in to Frutigen. The bus is on the standard BLS 330 timetable.